That One Rule

"Proponents [of the Duckworth-Lewis method] assured us that this was the fairest way of determining the outcome of rain-affected matches. Fans without calculators and computer printouts were not so sure."

- 2003 Cricket World Cup highlights DVD

Games require rules. Even Calvinball has one. Most of the rules are simple, especially in simple games, like tag. But then, there's That One Rule.

It's the one exception, it's complicated, it can get most people arguing over how long players can stay on base without getting tagged.

Whether these rules are caused by an Obvious Rule Patch or just a bad design decision, most players (or fans, in the case of sports) learning the game will wind up confused by this rule; particularly spectacular examples will confuse (and be hated by) advanced players.

The point here is that this is a case where most of the rules are very easy to understand except for a rule, or a few rules, that are glaringly complicated.

Differs from Loads and Loads of Rules in that this is a localized case of the problem. See also Grappling with Grappling Rules, an example from tabletop RPGs. Not to be confused with Scrappy Mechanic, which is not about a game rule or mechanic being complex or confusing, but about it being outright hated. Note that a rule can be both (for example, the draw rule in chess, below).

Board and tabletop games

 * Castling and en passant capture are confusing to many Chess players. The latter move is an Obvious Rule Patch, while the former is the only way in the game to have two pieces of the same color move at once and has highly unusual restrictions on when it can be used. Tournament Players will certainly be familiar with both of those moves; what frustrates them instead are the long-evolving rules about when they are allowed to claim a position as drawn.
 * Once or twice, chess grandmasters have demonstrated a lack of understanding about castling (namely, thinking that the rook is not allowed to pass through a threat, when that only applies to the king).
 * Some of the mechanics of Magic: The Gathering are like this.
 * Banding is a particularly bad example, though it is not too complicated, just occasionally creating counterintuitive situations because a group of creatures behaved in an unusual manner for blocking purposes, damage assignment (spells and combat damage were assigned to banded creatures in different ways, which confused players), and sometimes lead to odd situations that confused newbies (such as assigning damage to a creature which ordinarily could not even be blocked by the creature it is taking damage from, and in some cases, this would also prevent the creature from dealing any damage to the band at all). Bands With X, however, was completely baffling as originally written. X was a creature type or a description, such as "green Legends" or "Wolves of the Hunt". Why is this confusing? Because, despite its name, this did not allow these creatures to form a band with, say, Wolves of the Hunt, but with other creatures with banding or "Bands with Wolves of the Hunt". So, if a Wolves of the Hunt did not have Bands with Wolves of the Hunt somehow (possible in many ways), it would actually be impossible for another creature with "Bands with Wolves of the Hunt" to band with it! Both abilities were quickly dropped from the game.
 * A few old-school cards have very complex rulings because they were made before making sure that there was no room for interpretation became one of the game's priorities; see for example Ice Cauldron or Word of Command. However, the absolute worst part of the rules is what happens when there are multiple persistent abilities that affect what a card can do, epitomized by the interaction between Humility and Opalescence. Witness the block of rule clarification on the interaction between those two cards specifically, as well as how many times those rulings have changed.
 * The so-called "infinity rule" is also responsible for plenty of headaches. For example, if one player can do an arbitrarily large amount of damage, and another player can prevent an arbitrarily large amount of damage, things get ugly. (Would you believe that the outcome depends on whose turn it is?)
 * Well not if one infinity is countable and the other isn't. Remember Cantor's diagonal argument to determine which is which.
 * Summoning conditions in Yu-Gi-Oh! in regards to reviving monsters. If a monster's effect says it can only be special summoned one way, can it be special summoned from the graveyard after being special summoned through that method? For some cards (Dark Necrofear, ritual monsters, most fusion monsters), yes. For others (Armed Dragon LVs 7 and 10, Rainbow Dragon, most Elemental Hero fusions), no.
 * The fine print on spell/trap/effect monster cards can also lead to some unusual circumstances and headaches. Legendary Fisherman on the Field while "Umi" is active? If no other monsters are on the field, opponent can attack your life points directly. Trap Card negating the effects of all monsters on the field? Monsters can still be special-summoned by their own effect, ignoring any change to ATK and DEF as a result. Both of these are official rulings. Also, flip effects of monsters ONLY activate after damage calculation. A face-down defense-position monster that is attacked is NOT flip-summoned, it is merely "flipped". A handful of cards have the exact text "When this card is flip summoned..."
 * The most hated rule is easily the missing the timing rule. The fact that such a rule doesn't exist in the anime doesn't help.
 * The board game of go has extremely simple rules... except for the "ko rule" which is designed to prevent repeating positions. In the simplest form it just disallows one specific position which is sufficient for 99.9% of all games. The Ing Ko Rule resolves these .1%, at the expense of pages and pages of explication.
 * In World of Warcraft the armor penetration stat ended up so confusing and defining some classes to such a degree that as of the Cataclysm expansion it's being removed from the game entirely. Armor penetration was of course distinct from but interacted with abilities that reduce enemy armor, abilities that bypass enemy armor, and amount of enemy armor.
 * Another problem with Armor Penetration was that its effectiveness increased exponentially, making it vastly superior to other stats for physical damage dealers.
 * Monopoly has a few, and perhaps more than any other board game has families just ignoring rules they don't like and making up rules they do. A lot of people don't even know the mortgage interest rules exist. More think that the auction rule is fake!
 * Star Wars Customizable Card Game: Attrition. For a vastly simplified explanation, at the end of most larger battles, both sides are assessed a penalty, in addition to the penalty paid for losing a battle, which can only be paid by discarding combatants (as opposed to discarding from one's hand or deck), which counts simultaneously toward the penalties paid by the loser; this penalty or its remainder is often waived if the characters remaining have sufficient Plot Armor, but how much plot armor is needed depends on the whole penalty, regardless of how many has to be paid by Red Shirts, and the loser's penalty remains if it's not paid by the time remaining attrition is waived, and can, if the player wishes, be paid by discarding these characters. Even in a game notorious for Loads and Loads of Rules, the complications that would crop up around this one in particular are legendary.
 * Before 4th edition Dungeons and Dragons, initiating a grappling attack is usually cause for your entire gaming group to throw large, heavy objects at you. There's a reason the trope is call Grappling with Grappling Rules.
 * Submitting dragons in early editions required the GM to recalculate what percentage of the dragon's HP you had burned through with non-lethal damage and then make percentile roles. Worse, it basically amounted to giving a rampaging, roaring engine of death a blanket party and hoping it decided to cry. Bad rules are bad; violating the Rule of Cool and Rule of Fun at the same time is unforgivable.

General

 * The Salary Cap, in virtually any sport that employs one... and those without it have the That One Unofficial Rule that says teams in the biggest markets can just outspend everyone. Certain cap schemes are more complicated than others, at least in American sports:
 * The NFL's scheme is pretty simple: teams can't go over the cap for any reason; if they do, they're forced to start cutting players until they're below it. A player's signing bonus counts against the cap equally across every year of his deal, e.g. a $5 million bonus on a 5-year deal would count $1 million against the cap each year even if it was all paid in the first year. The player's yearly salary counts at full value. If a player is cut before the contract ends (and before the regular season starts), then the rest of the contract's regular salary doesn't count, but the remaining signing bonus cap hit applies for that year.
 * The NHL's scheme is a bit more complicated. Regular salaries can't go over the cap, but performance bonuses can go a set amount over without a team being punished. The yearly cap hit of a contract is the average of that contract's yearly salary. No individual contract can be more than 20% of the total cap. Players sent to the minors don't have their contracts count against the cap. Injured players' contracts do count, but a team can sign any number of players whose cap hit is at most the injured player's hit without penalty. A player can't be outright cut; they can be bought out for 1/3 of their remaining salary if they're under 28, 2/3 if they're between 28 and 35, or not at all if they're over 35, and the buyout is spread over twice the remaining years on the contract.
 * And it's not quite that simple, either. Players over 35 count for their cap hit minus $100,000 if they're sent to the minors in the second or later year of a multi-year contract. The cap is re-calculated per day, so if you're under at the beginning of the year, you can spend more later. Contracts also have limits on how much they can change year-to-year (half of the lesser value of the first two years salaries is how much a contract can change value by each year). According to the CBA (the governing document for this stuff), players who are suspended don't count against the cap, but the Players' Association and the league agreed that was unintentional, so they ignore that rule (and didn't explicitly tell anyone else they were ignoring it until 2011). There are reasons teams keep messing up when they write contracts - it's complicated.
 * MLB has a luxury tax system; any team that spends over a certain amount has to pay an additional percentage of the overage to the league which gets redistributed to player benefits, growing the sport in developing nations, and the Industry Growth fund. A first-time violator in a 5-year period pays 22.5% of how much they exceeded the cap, a second-time violator 30%, and a third- or more-time violator 40%.
 * The NBA has the craziest cap rules. There are several exceptions that a team can use to get over the cap, including
 * The mid-level exception, a one-time exception for signing a player (or players) in the middle of the season for the average NBA salary.
 * Rookie scale exceptions for signing 1st-round picks.
 * Various exceptions for a team re-signing their own players; these vary based on how long the player has been in the league.
 * The minimum player exception, where anyone signed to a league-minimum contract doesn't count against the cap.
 * The traded player exception. Ordinarily, a team over the cap cannot take on more than 125% of the salaries it's trading away. If a team makes a trade where the player(s) they get back count less against the cap than the ones being traded away, the salary difference "X" can be used in a future trade or trades up to one year later to acquire player(s) whose salaries are up to "X" more than they should normally be allowed to trade for. The exception can only be used in single-player trades.
 * The disabled player exception, which allows a team to replace an injured player with one whose salary is up to 50% of the disabled player's.
 * And in addition to all these exceptions, the NBA also has a luxury tax for teams that go a good deal beyond the soft cap, and the cost for exceeding the tax level is $1 to the league for every dollar over the luxury tax threshold.

American Football

 * Overtime in Collegiate American Football.
 * The computer formulas used to rank the BCS teams.
 * No, that's Serious Business, resulting in a book by respected sportscasters called "DEATH TO THE BCS!"
 * Most such rules are either buried in the rulebook until a controversy uncovers it (like the Tuck rule), or through subjective over-enforcement ("Defenseless Player" rulings or "Brady Rule" calls). The latter was wrongly attributed to Tom Brady; it's actually called the "Carson Palmer Rule", which was passed at the start of the 2006 NFL Season.

Association football (soccer)

 * The offside rule. The rule is of course very straightforward (basically, you can't score if you were behind all the defenders when the ball was passed to you); it's just tediously worded and has a reputation for being so complicated that newcomers declare themselves confused without even trying to understand, and those who understand it feel the need to explain it in as complicated and detailed a way as possible. This in turn gets people annoyed and, well, it merits That One Rule status, anyway.
 * There is one slight complication. You can't score if you're behind all the defenders but one, with the goalkeeper counting as a defender. This only matters when the play has gone past the defending goalie, but there's another defender between the player who got the ball and the goal. Which is to say, hardly ever.
 * The inability of anyone to explain the offside rule is gloriously parodied in The Little Book of Mornington Crescent. The explanation of Mornington Crescent's offside rule is half a page of dense, jargon-filled gobbledegook. Which concludes "This should not be confused with the offside rule".
 * The lack of goal line technology could perhaps be called That One Lack Of A Rule. Every fan knows the pain of having a good goal disqualified (or a bad goal by the opponent let through) thanks to a ref's poor vision. FIFA's refusal to consider change hit its nadir during the 2010 World Cup -- after news that fans in the stadiums were upset after watching ref screw-ups on big-screen monitors, FIFA's initial solution was a ban on in-stadium goal replays.
 * AFA (lit: Argentinian Soccer Association ) invented a rule where teams are downgraded from group A to group B according to an average, from all the matches played, of goals made and goals received. In the past it used to work the same as the rest of the world: the team with most defeats goes from Group A to Group B. Most people don't really get how this rule now works, and just watch the news to get how their team is going instead. The reason it exists is to keep big teams from getting downgraded.
 * Actually... that's not how it works. In most of the world, the team(s) with fewest points during a single season are relegated. In Argentina, the two teams with the lowest point average during the last three seasons are relegated. This benefits big teams because:
 * a) the big teams can be counted on being able to outspend most opposition to ensure a good season every three years, at least.
 * and b), the newly promoted teams have a harder time to keep playing in the top category, since they can't average a mediocre first season against previous ones like the big teams can. Newly promoted teams have very little margin for error.
 * Terribly subverted when River Plate, Argentina's biggest champion, ended up being relegated in 2011. Destruction ensued.
 * Similarly, the Brazilian championship of soccer had the tradition of always changing the formula. Sometimes yearly (and if a big team was meant to be relegated, there would be an Obvious Rule Patch to keep it in the top level). In 1999, the relegation was similar to Argentinian example above, based on the average points between 1998 and 1999. But It Got Worse when a team who suffered a last-minute downgrade started a lawsuit...

Australian Rules Football

 * The advantage rule.

Baseball

 * The infield fly rule. Much as with the offside rule above, the infield fly rule is simpler than its reputation -- an infielder can't deliberately let a fly ball drop in order to get an easy double play by picking off runners who would otherwise be forced to advance. If it's a double play situation and a batter hits a pop fly the umpire feels could be easily caught, then the batter-runner is automatically out and the force is negated . As long as there were less than two outs to start with.
 * The balk rule. Balks are relatively simple to understand, but hard to actually call. Balks are when the pitcher makes a motion towards a base, but throws to a different base. So a pitcher can't act like he's throwing a pitch and then throw to first base to pick off a runner, which is why when trying to pick off a runner, the pitcher will almost always move their front leg towards the base. The problem is, how do you classify a "motion"? Is it when the pitcher starts his windup? When he lifts his leg? When he moves his arm? Balks are almost always controversial at the Major League level.
 * The Posting System. Basically if a Japanese player wants to play in America, teams must first pay millions just to talk to him. In addition only one team is allowed the privilege of talking to the player. They get 30 days to negotiate a contract or the player must return to Japan (in which case the posting fee is refunded). The system actually would allow a team to promise more than a rival who could really use the player, with no intention of signing him, just to prevent said team from getting him -- and pay nothing.

Basketball

 * The NBA once had a technical foul for "illegal defense". Zone defenses were disallowed, since it allowed centers to simply camp under the basket. After a few years without this rule, they decided to make a rule that disallowed defensive players to be in the key (the painted area directly underneath and in front of the basket) for more than 3 seconds... which matched up nicely with the same rule for offensive players.

Cricket

 * The Duckworth-Lewis Method was devised as a totally fair way to decide matches affected by rain. Unfortunately it's an extremely complex mathematical formula the results of which change every time a ball is bowled, a run is scored, basically every time anything at all happens. Since its introduction, matches (one of them famously a World Cup semifinal) have been decided by one team's players and/or coach misinterpreting the results table and settling for fewer runs than they in fact needed.
 * There are those, of course, who argue that the whole of cricket is in fact a case of Loads and Loads of Rules and it's difficult to deny... but Duckworth-Lewis is infamous even among those who understand everything else perfectly.
 * The LBW (Leg Before Wicket) rule is well-known for being one of those cases where the judgement call is generally in favour of whichever cricketer is the best boxer.
 * Like many of these examples, the concept is straightforward: the batsman can't use his leg to block the ball, so if that's the only thing stopping it from hitting the wicket, he's out. In practice...
 * In Stephen Baxter's Time's Eye, British soldiers from colonial-era India try to explain cricket to the army of Alexander the Great. They manage to get most of it across with gestures and broken Greek. They give up trying to explain Leg Before Wicket.

Fencing

 * Stop hits and time hits in fencing are in theory incredibly simple, as they are counterattacks which retain priority over the initial attack because the initial attack is too "long-winded" and so it was interrupted. For example, if A's attack is legal, but consists of four movements, B can stop hit A by counterattacking during that fancy, silly wind-up. Good luck with getting fencers and judges to agree on what is a stop-hit and what doesn't count in the heat of swift-moving competition.

Ice Hockey

 * Buffalo Sabres fans are the only hockey fans that know the rules on players in the goal crease, the result of Brett Hull scoring a scoring a controversial Cup-winning goal off his own rebound in the third overtime period of Game 6 of the Stanley Cup finals. Video replay showed that Hull's skate was in the crease (i.e. the area in front of the goal, reserved for the goalie), which the Sabres argued was a violation of a rule then in effect that disallowed goals if an offensive player was in the goal crease. However, the rule stated that a player can enter the crease, as long as he has control of the puck, and the refs ruled that since Brett's shot rebounded to him, he had never lost control of the puck.
 * And, after that disaster, the rules were changed, so that the player could now be in the goal crease, as long as they do not touch the goaltender. This led to some angry goaltenders as opposed to some angry Sabres fans.
 * Icings are easy to spot and deal with. Conversely, icings are hard as hell to accurately explain (at least until the rule was changed).

Politics
Politics, like games, generally operates under a system of rules, and in many countries--especially liberal representative democracies--most of the rules are simple enough to be taught to children. However, even the simplest systems can get into a snarl now and then.
 * From the American Political System, we have:
 * The Twenty-fifth Amendment, specifically Section 4 (the part that gives the Cabinet the power to declare the President incapacitated), whose complexities are so interesting for writers, we gave it a trope.
 * The Electoral College. This one is relatively simple to explain and extremely easy to adjudicate, but it still gets Americans in a tizzy every time it becomes relevant. The Constitution stipulates that the President and Vice President are elected by an absolute majority of a special body called the Electoral College, composed of electors chosen in each state in a manner of the state legislature's choosing. Each state gets a number of electoral votes equal to its representation in both houses of Congress, i.e., the size of its House of Representatives delegation plus two (for its Senators); this gives us 535 electors for today's Congress. The Twenty-third Amendment grants Washington DC three electoral votes, giving us 538 electors. To win an absolute majority and thus the presidency, you thus need 270 electoral votes. Simple enough, right? But of course, the states have all chosen to choose their electors by popular election--i.e., the electors are chosen as slates of individuals honor-bound to vote for the candidate of the people of that state's choosing. In most states, the slates are chosen on a winner-take-all basis: whoever gets the most votes wins all of that state's electoral votes, even if it's not a majority. Even if you win a plurality in, oh I dunno...Florida by 537 votes out of nearly 6 million cast, you come home with all of Florida's 25 (at the time) votes, and your opponent, none. Most of the time, the person who wins the most "popular" votes (i.e. the total number of people nationwide who voted for him/her) also wins the electoral vote and therefore the Presidency, there are rare occasions when these do not line up...as with the famous Tale of Dubya, above. Americans get extremely confused by this, the news has to spend hours explaining the process, an explanation that often only makes people angrier. And don't tell anyone what happens if no-one gets 270 electoral votes...
 * And then there's Party delegates in the Presidential nomination process. One of the reasons Barack Obama won in 2008 was that his campaign understood the Democratic delegate system, and Hillary Clinton's did not.
 * The 2012 Republican Party primary has a lot of states giving electorates by percentage (i.e. if you win 75% of a state, you get 75% of the delegates). This is being seen as problematic because it has contributed to the dragged-out nature of the primary and Republicans are worried about how much the candidates are needling each other. Other people are just bored with how much news coverage it's getting.