Not the Intended Use

So, you've got an ability. And it has an intended use. You know this. But, just for once, you try to use it some other way. And it works. You've just discovered Not the Intended Use. Usually a Game Breaker, it often leads to Gameplay Derailment. Happens more often than you think. Contrast Useless Useful Spell.

Card Games

 * Dominion has the Chapel card; intended to get rid of hurtful curse cards, people realized it could be used to streamline your deck by trashing the low-value cards you start with.
 * Later deckbuilding games such as Puzzle Strike and Ascension use this as an intentional design choice, so removing your own cards from the deck for greater efficiency is now expected. Ascension doesn't even have any harmful cards to remove.
 * Not the Intended Use...Where would Magic: The Gathering be without you?
 * Unsummon to save your own creatures.
 * Unstable Mutation to wipe out your opponent's creatures. Or Immolation. Or Bloodlust.
 * Countering your own spells...just to reduce the number of cards in your hand (because of Black Vise).
 * The Millstone/Visions combo. And to a lesser extent the Fireball/Reverse Damage combo.
 * Cards that require you pay life...just so you'll get your Avatar of Hope in play.
 * Using Swords to Plowshares on your Killer Bees or Carrion Ants to gain a lot of life.
 * Ornithopter/aura combos in general.
 * Tearing up Chaos Orb in order to destroy the enemies permanents. This even sparked the creation of a "Chaos Confetti" card.
 * Magic doesn't really have "intended use". It just has obvious use.
 * The Yu-Gi-Oh! card game has more than a few instances of this too:
 * Barrel Behind the Door was initially meant to bounce back damage done to you by effects. It works just as well bouncing non-cost damage from your cards to your opponent.
 * Using Armageddon Knight's ability to negate your own effect, which destroys him... so you to summon a quite powerful Meklord Emporer.
 * Munchkin allows you to use "Go Up A Level" cards on your opponent in order to force them to fight/run away from a monster that would ignore them if they were just one level lower.
 * The creators of the game were asked if the cards could be used that way--while that hadn't been the intent, the creators responded it was such a Munchkinly thing to do, they just couldn't say no.
 * In the Pokémon Trading Card Game, Claydol's "Cosmic Power" is meant to draw cards from the deck, but its secondary effect, putting 2 cards back to the bottom of the deck, can also be used to prevent a player from running out of cards in one's deck and thus avert a loss by decking out.

Live Action TV

 * The entire premise behind MacGyver.
 * Ditto for Mythbusters.

Music

 * Scratching records in hip-hop.

Sports

 * Time outs in several sports including Basketball and Lacrosse. Intended to allow teams to meet and plan strategy. Can be used tactically to prevent a player from losing possession and restart play in a controlled manner.
 * Football, too. The most common use for a timeout is to stop the clock, and the second most common is to avoid a penalty for delay of game or too many men. It's far rarer for a timeout to be called to talk strategy on a critical play.
 * Somewhat less common is the use of a timeout to "freeze the kicker". If your opponent is about to kick a field goal, calling a time out just before the play starts will often mess up a kicker's timing enough to cause him to miss the field goal when he does eventually kick it. This is only done occasionally, however, because in most circumstances time outs are much more valuable for the above-mentioned unintended uses than for this one.

Video Games

 * Final Fantasy:
 * Another novel strategy to Final Fantasy is to hit yourself to cure Sleep or Confuse, as opposed to waiting for your opponent to hit you.
 * Also, in some of the games, if you have a frozen character, casting any fire spell will thaw the character. Especially nice if you have an armor/accessory that absorbs fire magic.
 * Using the Swap spell to turn you into a statistical Physical God in Final Fantasy II. Its intended use was likely for emergency HP/MP refill purposes.
 * In Final Fantasy IV, an enemy casts Reflect on you so you can't heal yourself. You'd better learn that little trick, because Asura heals herself after every attack you land.
 * Reflect can also be used offensively by casting it on yourself. Enemy has Reflect status, forcing you to hold off from powerful magic attacks? Reflect yourself and then cast devastating magic on yourself that reflects and hits the boss. In some Final Fantasy games, a Reflected spell actually does a lot more damage than if it hit normally, so the best strategy in many situations is to Reflect a party member and bombard the boss with spells. Note that this makes it impossible for you to heal yourself with healing magic, so you need to Reflect the boss and throw heals on him if you want to heal yourself.
 * You can also cast Reflect on the whole party and hit them with an aoe effect, resulting in multiple hits to the opponent.
 * In the DS version, the Adrenaline augment doubles the damage a character inflicts when he or she is at critical health. Casting Tornado on a character with Adrenaline is an easy way to satisfy this condition. This is particularly useful on Rydia who can barely take hits anyway.
 * Final Fantasy VI had Vanish, which makes you immune to physical attacks. Nice, but it makes the next magic attack hit. This led to the Vanish/Doom combo, an instant death attack that will even defeat anything that doesn't have a Vanish immunity! (Which, as you can probably imagine, isn't much.) Patched in the Game Boy Advance version, and careful examination of other status spells (which still fail on a Vanished immune target) indicates that it was a bug to begin with, but still intuitive.
 * The Throw Stone ability in Final Fantasy Tactics lets you build up Job Points. Otherwise, it's just useless damage at long range. Ditto with Accumulate/Build Power. The +1 to physical attack is meaningless unless you do it a lot, and most enemies really won't let you do it a lot. But leave one enemy alive, and send all your units running around the battlefield spamming the ability, and hello job points! In fact, there's a lot of abilities that might be useful, but are much better at building job points.
 * A particularly well-known example in Quake and many other FPS games is provided by the rocket launcher. Its intended use is of course to make Ludicrous Gibs of groups of enemies. Many players instead choose to use it to make massive Sequence Breaking leaps. Rocket jumping became an Ascended Glitch for the FPS genre - the Soldier in Team Fortress 2 is designed for just that.
 * In The Legend of Zelda, there are a few instances where this pops up:
 * In A Link to The Past, using the butterfly net against Agahnim will bounce his spells back at him.
 * In Ocarina of Time, using a bottle against Ganondorf will bounce his magic attacks back at him. This also works in Wind Waker against Phantom Ganon, and against in Twilight Princess.
 * In Links Awakening you can use the shovel against Dethl to deflect his fireballs when he assumes Agahnim's form.
 * In Twilight Princess, using the fishing rod will distract Ganondorf, opening him up for a free hit.
 * In Skyward Sword, The bug net can distract, the Final Boss.
 * In Metal Gear Solid Peace Walker, the Sling Post's intended and primary use is as a key component to the Human Slingshot (hence the name). However, the players can also use it in single player mode to knock the enemy senseless. Miller even Lampshades the trope by calling the player when doing this and saying "That's not what its used for!"
 * League of Legends had the Innerverating Locket. This item restored some of your mana and caused a minor self-targeted aoe heal whenever you used one of your abilities. Originally it was thought of as a powerup for healers. Then people realized you could use it on Udyr (a melee tank who uses his abilities in quick succession.) The Locket/Udyr build was so powerful that it forced Riot to remove the locket from the game.
 * Another example is Tear of the Goddess. This item gave you a very large mana pool over time. It was originally thought of as a caster item. Then people started to use goddess tear on other characters. Riot took note of this and made the Manamune, which is an item for DPS characters that builds from Tear of the Goddess.
 * While radically different uses for champions are occasionally found, the champion Gragas stands out for the alternate having massively surpassed the original. He was intended as a tanky melee fighter, using his abilities mostly for utility to debuff, disrupt, and initiate. He is basically never used this way, instead functioning as a burst mage who relies on his tank origins to make him tougher than most mages. For a long time the Riot Games recommended items focused on completely different attributes than most player recommended builds, until they eventually gave in and completely overhauled them.
 * Sion a giant berserker zombie with a massive axe, he was obviously intended to function as a beefy melee DPS, using his Death's Caress (an exploding shield) for protection in fights and his Cryptic Gaze (a ranged damage spell with a lengthy stun) to catch enemies for him to wail on. While he is often used like this, people noticed that both of those skills had high base damage numbers and perfect 1-1 ability power scaling ratios and he became one of the most powerful burst mages in the game. The scaling was later decreased to decrease make this less prominent.
 * Elder Scrolls 3: Morrowind featured Alchemy, allowing you to craft potions. The intended use was to craft potions that would improve your skills in combat and dialogue... but it was discovered that by making and drinking an Intelligence potion, you could make better Intelligence potions which you could then drink. After a few iterations, you could easily make potions that made you a Physical God.

MMORPGs

 * The flee ability in Warhammer Online is most of the time used to get from point A to point B faster.
 * Ditto for the Charge and similar abilities in Guild Wars.
 * Perfect World: the Cube of Fate can be entered via any major city, but when you exit it, you end up in Archosaur, no matter which city you entered from. As such, it is most often used as a free teleport to Archosaur.