Game Breaker/Fallout

Fallout 1 and 2

 * If your Gambling Skill is at 100% or higher, you almost can`t lose at roulette if you keep placing low bets (about 2$). All you need to do is to hold down the hotkey '1' in the dialogue at the table and wait until you're in the millions. Since everyone except the casinos have limited resources, the whole wasteland economy is somewhat broken.
 * Even with a pitiful Barter skill level, other people's goods will be worth substantially less than yours. Thus, with a bit of swapping back and forth you can take everything a person is willing to sell you for free. Later games fixed this by jacking up the merchants' selling prices so that even with a maxed barter skill you still pay through the nose for items. However in all games it's possible to shoot the merchant in the face and take all their items for free - sometimes without even being attacked by guards for doing so!
 * The perk you gain after winning the New Reno boxing tournament reduces normal damage (anything except laser, plasma, fire, explosives, electrical and EMP) by 12 points, which adds to any armor you wear. The calculation is made for each bullet (you can get hit without armor by a minigun blast for no damage), and the only endgame enemies who use laser or plasma are non-military personnel (low HP and no armor) and the end boss.
 * Abilities that reduce the AP cost of attacks are seriously unbalanced. There is a trait that reduces the AP cost of all attacks by 1, but prevents the character from making aimed attacks. Combine this with perks that also reduce attack AP cost, and by the time you're around level ten, turbo plasma rifle shots cost 2 AP, and super sledge attacks cost just 1. With high agility you can do 200+ damage per turn with no special preparation, which is enough to kill everything but a few bosses in one turn, or shred groups of super mutants.
 * The Bozar in Fallout 2 has the power of a minigun, the accuracy of an assault rifle, and an ammo type which is cheap to purchase and sold in bulk just about everywhere. A group of shop guards you can meet fairly early on have the gun conveniently unequipped in their inventory. All of them are unusually easy to steal from, even with no points in the Steal skill. Commandeer two of these guns and stick them on a character with a decent skill in Big Guns and you have more than enough firepower to last throughout the rest of the game.
 * Also in Fallout 2, with a bit of saving and luck you can run straight to an Enclave which contains the second best armor in the game, for free. And you don't even need to kill anyone.. Since it gives a +4 strength bonus, this is completely ridiculous.
 * By knowing where to look and a small ammount of reloading, a character can go from 6 points of luck to 10. Combine this with Finess and you get a total critical chance that will likely exceed 20%. Add in a P90 (every bullet can score a critical, every critical will have a HIGH chance to instantly kill a target) and the game is over barring a few more hours of mouse clicking. Being aware of the ways to boost statistics in this game also means you can get 10 ability points in EVERY SKILL when factoring in item bonuses.
 * Speaking of statistics, a player can have one companion for every point of charisma over five. The check for this is based on your CURRENT charisma score instead of your actual one. This means that by tripping on mentats, a character so dumb he cannot normally understand human speech can have a loyal following of five lethally powerful companions who fill out all his dump stats and leave him with barely enough living enemies at the end of the first combat round of any encounter to get a punch in.

Fallout 3

 * The friendly and intelligent Super Mutant Fawkes, whom you can rescue in the main questline. Later, he can team up with you if you have good Karma. He is Nigh Invulnerable, wields an incredibly powerful gatling laser, and he steals so many kills that most players have to fire him until and unless they've hit the level cap.
 * The Tesla Cannon gained at the end of Broken Steel uses very common ammo and is (almost) a guaranteed 1-hit-kill, with area damage in case you miss.
 * The Chinese Stealth Armor from Operation Anchorage makes you nigh invincible, provided you have decent stealth (about 80). It allows you to sneak up to an enemy, from the front, set them on fire with a flaming sword, be pushed around by them, shoot them with little darts and plant explosives on them in full daylight without ever being detected. Thanks to the automatic critical when attacking undetected, most foes go down from the first hit. Got detected anyway? Retreat, wait a few seconds, be hidden again. Add to this that nobody gets alarmed by their friends suddenly going up in a blaze (more criticals), and you have the ultimate armor.
 * The Grim Reaper's Sprint perk restores all AP if you kill an enemy while using VATS. Now consider that with decent stats it's ridiculously easy to kill at least one enemy in each VATS cycle; also consider that while VATS lasts you're effectively invulnerable. Keep hitting the VATS key as you exit it, and you'll slaughter every enemy in range while scarcely receiving any damage yourself.
 * There's a setup that turns every single standard attack into a critical hit. Deathclaw Gauntlets do 20 damage per hit, a critical hit gives an additional 30 damage. As previously mentioned, they ignore armor, so they'll always do the full critical damage every single hit.
 * Several of the unique items that comes from the Monty Haul that is Operation: Anchorage:
 * The Winterized T51b Power Armor. Not the strongest armor in the game, but the developers accidentally put in the nigh-indestrucible simulation version - odds are it'll never drop a single point of DR in an entire playthrough.
 * The Chinese Stealth Armor. Repairable with Recon Armor and gives a permanent stealth field while sneaking.
 * The Gauss Rifle. Essentially an Energy Weapons sniper rifle, it has high damage, uses the common microfusion cells, and knocks down enemies on critical hits. Oh, and it has a critical multiplier of five.
 * It's also worth noting that a particularly sneaky player can place a corpse into the VR pod (in a seated position, which can be tricky to arrange), allowing them to place all their virtual equipment into the corpse's inventory before the game takes it away as you're waking up. The game will search your inventory for virtual items just a few seconds later and find nothing, allowing you to loot the corpse for all your invincible gear. Using this trick along with shooting and stealing Jing Wei's Shocksword out of his hand gets you an unbreakable set of armor (with some items you can't get any other way) and Shocksword which does electricity damage on hit.
 * The steel ingot rewards from The Pitt are several powerful unique items, but two are worth special mention:
 * The Steel Knuckles, having the lowest AP cost of any weapon in the game. Combined with Paralyzing Palm, even Deathclaws are helpless in melee.
 * The Perforator assault rifle beats out every other assault rifle in accuracy, comes with a scope to boot, and is silenced, allowing you to punch holes in enemies from a distance.
 * From a simple side-quest (or Sequence Breaking) in Point Lookout, you can get the Backwater Rifle, a unique lever-action rifle. While Lincoln's Repeater barely beats it in terms of standard damage and clip capacity, the Backwater Rifle uses the very common 10mm ammo as opposed to .44 magnum and has a critical multiplier of five as opposed to two.
 * Alien epoxy from Mothership Zeta. It restores a fixed percentage of the durability on your equipped weapon based on your Repair skill - even weapons that can only be repaired by merchants. There are over a hundred throughout the Mothership and you can get an infinite amount after the expansion questline is completed.
 * Even one of the basic alien weapons is a Game Breaker: the Alien Disintegrator. It packs a punch while having a clip size of 100 shots and literally reloads with a mere press of a button. You'll have thousands of shots if you save up throughout the expansion.

Fallout New Vegas
The game is chock-full of Game Breaker weapons and armor that can make quests and enemy encounters a breeze, even from the start of the game.


 * Walk a short distance away from Novac to the Broc Flower Cave and you'll find the Ratslayer, an alternate version of the common Varmint Rifle that's stacked with mods, including an expanded magazine, suppressor and day/night scope, plus a 5x Critical Hit multiplier. It's ridiculously helpful during your early adventures.
 * Due to a scripting error, Boone (the NCR sniper) has a 'more than 100% critical hit ratio, which makes enemy encounters in the early portions of the game insultingly easy. Not only does his hunting rifle deal the same damage as a sniper rifle, but his Spotter perk allows you to paint targets (which then leads to him getting continual headshots), and his health is large enough to allow him to continually engage enemies for an extended period of time before fainting. In addition, due to a glitch regarding armor with stats, Boone will gain a perception bonus every time he switches equipment...and they stack, giving him a possible max perception bonus that will continue indefinitely until you remove his beret. The only way to not break the game is to purposely avoid doing his recruitment mission.
 * Normally, you will lose Karma for stealing ammunition or items. However, it's possible to take a bucket with you into a store or stronghold and liberally loot to your heart's content! Picking up ammo as an object and placing it into a bucket, then dragging it to a secluded location will allow you to steal everything without losing Karma. You're only limited by the size of the container you use to steal.
 * Shotgun Slug ammo. Normally, shotguns do a fair amount of damage divided into several small bullets (around 7). This means that, once you start facing armored opponents, your shots won't do diddly. However, slugs condense that damage into a singe shot that does the same total amount of damage. Even better, with the perk Shotgun Surgeon, your shots ignore 10 DT, meaning that anything wearing light armor may as well be naked. The higher level shotguns, in particular, the hunting shotgun and it's unique variant, Dinner Bell, can take out most any human target (excluding Lanius) with only a couple of shots at most. With Gun Runners Arsenal, you can buy Pulse Slugs, which do less damage, but are extremely damaging to robots and power armor. Combine this with Center Of Mass and Bloody Mess, and you can reduce enemies to a pile of bloody chunks that are no longer identifiable as human remains.
 * It really says something when, once you get the Gun Runners Arsenal DLC, the only weapon category that received no unique varients or new upgrades/mods were the shotguns.
 * From the Dead Money DLC:
 * The lovely little gem known as the Holorifle. This is a scoped energy rifle that is, with Max Charge cells, outdone only by the Gauss rifles, but unlike Gauss rifles, a shot only requires one round of ammo, not four or five. Plus, it can be upgraded to do more damage, with reduced spread and improved condition. It also performs a damage over time effect to any enemy it hits. And while it can only be repaired by NPC's or weapon repair kits, at 100% condition, standard ammo causes no degradation. Use Overcharge or Max charge, though, and it takes damage. More importantly, it isn't affected by the Commando perk, or any of the energy weapons perks. While this sounds bad at first, this means that you DON'T need to take these perks if you're going to be a holorifle based character, freeing up several perks for character growth that you otherwise would not have available. The best part? You don't have to hunt for this weapon, you receive it at the start of Dead Money. The only thing you have to hunt for are the mods, two of which are located in areas you have to go to complete the storyline.
 * The player can obtain nigh-infinite Stimpaks and Weapon Repair kits by converting Sierra Madre chips at vending machines. Completing the expansion pack (only) after winning 7500 at the casino will give you 1100 chips every 3 days. Stimpaks are 25 chips each, and Weapon Repair packs are 20. For those of you not wanting to do the maths: 44 Stimpaks. 55 Repair Kits. For free. Per three days.
 * The gold bars. In theory, the player should not be able to carry more than 1 or 2 (at the most) out of the vault because they're so heavy, and they're supposed to represent letting go of greed (unlike Elijah, who will stop at nothing to get inside the vault). However, there's nothing stopping the player from either carting gold bricks back to the area's entrance or using a Stealth Boy to walk past Elijah when he comes down to the vault. Getting away with all the gold will net you 389,943 caps - it's impossible to find enough vendors in the entire game (DLC's and all) who have that many collective caps, let alone enough to buy a single bar off the player. However, the gold bars actually have a rather unimpressive money/weight ratio, and actually acquiring them requires unreasonable amounts of time. You're better off sniping Fiends (which is also more fun).
 * They look mighty nice strewn across your Presidential Suite, though. (The only catch is that you can't lift gold bars-- likely intended as a wise precaution to prevent savvy players from simply carrying the gold out in some kind of container. Not that this stopped anyone from doing it the old fashioned way.)
 * From the Old World Blues DLC:
 * The Stealth Suit Mk. II is the best stealth armor in the whole game. Perform all four optional tests in the X-13 research facility, and you'll receive a suit that gives you +25 Sneak, +1 Perception, +1 Agility and +20% movement speed while crouching, plus auto-stimpak injection. The only way not to get this is by deliberately ignoring it. Combine it with a weapon that does ridiculous sneak attack criticals (like the aforementioned Ratslayer from the main game) and enemies won't know what hit them.
 * The downside to the Mk II armor (besides the mildly condescending and immediately annoying on-board AI voice) is that it doesn't just inject you with Stims... it hits you with constant Med-X too, which you can get addicted to. Addiction is a real pain in the ass, which is Truth in Fiction.
 * From the Lonesome Road DLC:
 * is a deliberate Game Breaker given to you right at the start of the mission. Most notably, after collecting two circuit boards (one found in the same complex where was activated), you will receive the option to have your weapon repaired by 25% once a day (thereby saving you thousands upon thousands of caps in repair fees), and you'll get free ammo every day (which, depending on your Speech or Repair skill, will net you Energy/Microfusion cells, flamer fuel or satchel charges). You'll never have to worry about weapon degradation or running out of ammo for the duration of the DLC. The best part, however, is that all these benefits  - have fun!
 * The best rocket launcher in the game, Red Glare, is more or less handed to you a third of the way through your quest, and three mods for it are sold at every Commissary in the Divide. If you've been saving your caps (or if you decide to abuse the Commissary's replenish stash of caps), you can get all three mods as soon as you get it, and use the fully-upgraded auto launcher with the best zoom (4.35x) in the game immediately.
 * Turbo, a rare chem which creates a "bullet-time" effect that slows down everything except for the player, allowing you to dodge any attack with minimal effort. It's not a game breaker by itself because the duration is normally very short, but with a certain combination of perks it is possible to get its duration up to a full 30 seconds (a full minute with one of the traits found on DLC)--more than enough for you to win any encounter with a single dose. The other balancing factor is that it is extremely rare, but this too can be overcome if you find the recipe, because the individual components are fairly common.
 * On the other hand, one of those components is a poison gland from a cazador. If you're Badass enough to farm cazadores, you don't need large quantities of Turbo, making the large-scale crafting thereof Awesome but Impractical.
 * Turbo's Game Breaker potential is limited by it's aforementioned rareness. The Implant GRX perk can be taken at level 30 and has the exact same effect as a dose of Turbo, albeit with a shorter duration. However, with the previously mentioned drug extending perks a single shot of GRX lasts 20-30 seconds, depending if you have rank one or two of the perk, more than enough to kill anything in the game, and if somehow you can't finish a fight up in time, feel free to take another FOUR to NINE doses (rank 1 or 2 respectively). That reset daily. Oh, and did I mention it's impossible to become addicted to it. It stays with you in Dead Money. The only real down side to basically being the flash with a gun is an 8 endurance entry fee.
 * The MF Hyperbreeder Alpha is ludicrously overpowered with a high energy weapon’s skill. It is essentially a pistol-sized Gatling laser with UNLIMITED AMMO that can be bought very early on if you know how to get it. When fully charged, the Hyperbreeder can disintegrate deathclaws. There is literally no reason ever to use any other gun in the game unless you are outnumbered something like ten-to-one by significantly higher leveled enemies.
 * Debatably a Infinity+1 Sword, PEW-PEW is easy to acquire fairly early in the game with a bit of knowledge and consuming every bottle of Sarsaparilla you come across. Sure, it eats ammo like a starving man devours fresh bread, and yeah it only can shoot two rounds before reloading. With this price however, comes an extremely easy to repair, improved holdout weapon, with insanely common ammo, that does some of the highest damage per shot, and with one of the highest crit multipliers and crit damages in the game. Even someone with no skill in energy weapons whatsoever can drop the highest level death claws in a few shots. Someone with a reasonably optimized energy weapons build, well... let's just say that those two cons I pointed out above no longer mater.