In a later version of the Tomb of Horrors, the scepter and crown of disintegration (put the crown on your head, touch one end of the scepter to it, you disintegrate) cannot be removed from the room they're in by any means (the description goes to great lengths to cover any eventuality). Earlier versions of the Tomb had no such rule at all. The reason eventually emerged during a conversation on a message board: One of the artists working on an earlier copy of the module was invited to a session of the Tomb DMed by none other than Gary Gygax himself. The artist took the scepter and crown from the room, then eventually placed the crown on the fake skull of Acererak and touched the scepter to it, disintegrating the lich instantly. Gygax was stunned, as the eventuality had never occurred to him. The artist, on the other hand, thought that's what they were there for. Gygax eventually ruled that the disintegration did occur, and complimented the artist's quick thinking. The artist was quite surprised when he was later informed of the rule change.
The rule that bonuses of the same types don't stack - only the largest one takes effect (with the exception of dodge bonuses to AC in third edition). This has led to many rule patching to give untyped bonuses types so they couldn't be so easily stacked.
3.0 spellcasters had a bad habit of using summoning heavy creatures in midair, causing them to deal obscene damage as falling objects when they hit opponents. While this is an eminently logical thing to do with a summoning spell, it's not only Not the Intended Use but also a Game-Breaker. Wizards of the Coast amended the summon spells in 3.5 to prevent creatures from being summoned into an environment that can't support them (i.e., no flying whales).
You can't sunder armor in 3.5. You can break weapons, shields, even items they're wearing like pendants. Just not armor. It would be easier to just break the fallen paladin's armor and then stab him, leading to silly situations such as the above.
Dimensional Door. In 3.5E it's pretty much an early teleport spell, in previous editions (as the name implies) it created a pair of portals through which the PCs could travel great distances. While that may not sound so bad, PCs often created horizontal or diagonal doors to bisect enemies (or fortifications!) that led to instant kills. Another tactic was to open a portal into a volcano or sea and use the exit portal to flood an enemy base with lava or drown it completely.
When a player turns into an animal, he can no longer speak, but can make that animal's natural vocalizations. The rules inform you that a parrot's natural call is just squawking, so no talking as a parrot.
A player can't use multiple magic rings on the same hand, or multiple amulets, because their proximity causes the magic to malfunction. A player can, on the other hand, wear a magic ring on each hand under magical gauntlets while wielding a magical two-handed sword.
Any monster that has the ability to swallow enemies whole has text in its description noting that a victim can cut their way out of the stomach, but "muscular action" forces the hole closed behind it. It's clear this was done to patch two issues: 1) the monster simply bleeding to death after a giant hole is cut in its belly, and 2) the swallow ability being rendered completely useless after the first time someone escapes.
Pathfinder is basically a tweaked Dungeons & Dragons 3.5 and (to make up for an initial shortage of material) was said to be compatible with 3.5 which led to some game breakers.
The Quick Draw feat allows you to draw any item from your pack as a free action... except flasks of alchemist's fire or acid. You also cannot sneak attack with such items, unlike all other weapons. These changes were obviously put in place due to volleys of flasks being popular among 3.5e rogues as a means to fight enemies resistant to physical damage or vulnerable to fire.
Also, the original 3.X rules for non-lethal damage resulted in jokes about how you can punch people all day without killing them, the rule was changed so that after a character has accumulated enough non-lethal damage to equal their maximum HP any further damage is automatically lethal.
It was ruled that, if an attack would do zero damage, instead of always doing one point of Scratch Damage, it does one point of nonlethal damage. Most creatures that were affected by this rule were creatures like housecats or rats, which were fairly notorious in 3.x for their ability to injure or defeat 1st-level humans (scratch damage is a big deal when you have four hit points) - humorously demonstrated here.
The Magus has the option to get a Swashbuckler deed and use their arcane pool points as panache points to use the ability. However, they count as a 0th-level Swashbuckler for the purposes of the ability. This is a highly unusual way to implement it (many abilities copied from another class would do nothing at all at 0th level), and had to be confirmed via errata - which added that although they can spend their arcane points as panache points, they don't have panache points for these abilities, not even if they get actual panache points by another source. The goal appears to be to stop the Magus from gaining Precise Strike or Evasive, respectively powerful offensive and defensive boosts that care about whether you have at least one panache point and your Swashbuckler level, that were considered acceptable on a non-caster melee class but too powerful on the Magus. This has the side effect of making the majority of Swashbuckler deeds, even quite innocuous ones, partially or entirely nonfunctional if taken by the Magus.
A popular defensive measure for Alchemists was to get a Tumor Familiar with the Protector archetype. This allows the Alchemist to shunt half the damage they take to a familiar which regains 5 HP each turn. Ultimate Wilderness created an obvious patch on this by preventing Tumor Familiars from taking the Protector Archetype, because, apparently, they are unable to be so loyal as to give their lives for their master. The fact that they literally have a feat called Die for your Master is surely irrelevant.
The Summoner class includes a note that the player's eidolon cannot wear armor because it "interferes with the Summoner's link to it." This is obviously just a patch to prevent all Summoners from making their eidolons humanoid-shaped so they can put them in armor and double their armor class.
The Ranger ability that let you make continual attacks until you miss was changed in the errata to have a 5-attack limit, as it was possible to make a build that had an almost zero chance of ever missing, even against the strongest monster in the Monster Manual.
The Ranger power "Unbalancing Parry" allowed the character to move an enemy behind them if they countered the enemy's attack. Unfortunately, the rules on forced movement specified that if the power didn't specifically state a maximum distance you could move an opponent, you could move them wherever you liked as long as they ended up on the square the power specified. This meant that the Ranger could "parry" the enemy through a tour of the entire battlefield, including impaling themselves on nearby spikes or entering fire hazards, as long as they ended up behind the Ranger.
A similar problem with forced movement arose with a rule that banned forcing creatures to move upwards. This was intended to prevent launching creatures upwards and leaving them to fall back down and suffer damage, but an errata had to be issued to state that the existence of a rising slope or staircase on the ground did not prevent all forced movement by requiring it to include an upward component.
Dungeons & Dragons 5th Edition changed how Actions work in combat, which led to a lot of questions from new and old players alike about how they worked (and why they were named they way they were). The spell Haste gives us a clear example of this trope: it gives you an extra Action during your turn, but it describes only a few specific things you can do with that Action, and if you attack you can't use abilities that let you attack more than once per action. Also, even though Haste is a spell that requires you to be a spellcaster to even use, you can't use the Action that Haste gives you to cast any spell, even the most basic ones.
In most D&D-like games, you can't wear more than one or two magical items of a certain "slot" and benefit from all their powers. While it makes sense that you can't wear multiple pairs of, say, boots, there's no reason for the usual "two rings, one amulet" rule other than balance issues. This is usually justified with a contrived excuse that the magic items will interfere with each other.
Construction rules often have arbitrary-seeming restrictions. For instance, conventional combat vehicles can use standard heat sinks just fine (and must account for all the heat their entire energy arsenal might build up if fired since they're not allowed to overheat), but are explicitly prohibited from using other types — notably double heat sinks, which in conjunction with their ability to already fire ballistic and missile weapons with no heat worries would allow fusion engine-equipped vehicles in particular to really load up on lightweight energy weapons and make them easily too powerful for a game where the BattleMech is supposed to be the "king of the battlefield".
There was also an instance where Battle Armor riding on an Omni Mech can be shot off of the 'mech by shots that land on the torso. Doesn't seem too bad, but given that there is no weight penalty for carrying Battle Armor, the Battle Armor were always the first to take hits, and the 'mech's torso wouldn't begin to take damage until all the Battle Armor were shot off... it's understandable why the next rulebook created fixed locations for each Battle Armor. (To clarify, this made Clan Elementals the equivalent of nearly 3.5 tons of standard armor at a cost of 1 ton of weight and a no-torso-weapons fire restriction. Now recall there are some Clan Omnis with no torso-mounted weapons.) This was further patched by changing it so that any time an attack struck a location on an Omnimech that was carrying Battle Armor, you roll to see whether the attack hits the Battle Armor or the mech. There's also a chance that the Battle Armor will automatically dismount the mech at the start of the next turn, too, due to them not being willing to simply sit there acting like an extra layer of ablative armor.
Early rules for Rotary autocannons, LBX autocannons and pulse lasers allowed them to use Targeting Computers to make called shots. Usually, calling shots comes at a penalty to the to-hit dice, but when it succeeds, it can cause some serious damage. 'Mechs carrying a Targeting Computer and one or more Rotary AC/5 could theoretically put 30 damage into one location, buzzsawing through all but the toughest armor in a single hit. 'Mechs carrying a Targeting Computer and pulse lasers or LBX autocannons could use the pulse lasers' to-hit bonus to offset the to-hit penalty and score fairly easy targeted hits, and much the same was true for the LBX cannons firing cluster rounds. The rules were soon patched to remove the ability for any weapon that fires multiple projectiles in a single salvo (including the pulse lasers, as they are fluffed as firing multiple laser beams in rapid succession) to make called shots.
Land-Air 'Mechs were originally a Master of None multirole machine, and the Powers That Be generally wanted them kept out of the setting and all game campaignsto avoid the wrath of theexcessively litigiousHarmony Gold. Early on, LAM units were generally regarded as inefficient under the basic (Level 1) rules and were not often used, which suited the developers just fine. Come the advent of Level 2 technology, however, Land-Air 'Mechs were suddenly more useful, being able to make up for the lost weight that LAM technology demanded and become viable battlefield units at long last. However, it took nearly two decades for the developers to realize that this meant that they could sneak back into the canon and board games, and they finally thought to patch the construction rules so that LAM units could not use any of the lightweight advanced technology introduced in Level 2 and 3 play to save weight, on grounds of 'interfering with their transformational abilities,' reducing them back down to inefficient curiosities instead of allowing them to become super-powered Jack-of-All-Stats that could make both conventional Battlemechs and Aerospace Fighters completely obsolete.
Another rule that took a long time to get put into the game was extending the tracking of heat past the 30-point cap. It used to be that heat wasn't tracked after it exceeded 30 points, which led to a lot of Min-MaxingmunchkinswhoringPPCs and large lasers minus the heat sinks to support them. The idea was to launch an Alpha Strike and deal heavy damage to a single target that maxed out the heat gauge, then shut down. Since these builds used only energy weapons with no ammo to explode, they could just wait until they managed to succeed on a restart roll, then do it all again (sometimes in as little as one turn). Now, any heat that exceeds 30 is tracked, and 'Mechs must vent all of their heat above 30 before they can try for a restart roll. Homebrew beasts with 6 ER PPCs that rack up 90 heat points per turn now need to vent 61 heat before they can even hope for an engine restart.
Protomechs have a "near miss" result on their hit location chart that negates the damage dealt. This was quickly revised so that it did not apply to area-effect weapons like bombs, artillery fire, or nuclear weapons so that it's no longer theoretically possible to dodge the effects of a close range detonation of a tactical nuke.
"Under no circumstances can any [necron] make more than one teleport move in a single turn... There are no exceptions to this, no matter how clever your logic."
"Please note that it is not possible to master-craft grenades!" note However, Dawn of War 2 has an item (and Space Marine a Perk) that disagrees with that rather blatantly. This despite the Holy "Orb" of Antioch featured in Codex: Black Templars, which is explicitly the creation of a "master artificer" in its description!
Space Marine drop pods are clearly 10-man craft (visible in the model and still stated in some codexes), but other codexes expanded it to 12 to allow an independent character to deploy with the squad. Without changing the model.
Later editions added a rule "if you have no units on the table at any given time, you automatically lose the game", to prevent the otherwise Game-Breaker tactic of deploying your entire army in reserve. This made Daemon armies a lot harder to use, however. A later rules patch removed their "must start with at least half of the army in reserve" rule to compensate.
Many of the Online FAQs were the result of the writers getting a little ahead of themselves and forgetting to state the obvious. The most infamous case was defining what a plasma weapon was, given that several weapons are explicitly stated to be plasma weapons, but do not carry the word in the name or any special rules related to other plasma weapons (several other weapons that were never stated to be plasma weapons were retroactively given this classification, causing much facepalming). Given that there is a competitive scene and general randomness of the game, these are a necessity as rule lawyers have a field day whenever a new codex comes out.
A rather infamous one regarded the use of Power Fists and Lightning Claws to get extra attacks. See both weapons are generally used in pairs (either a pair of fists or a pair of claws) but due to a quirk in the wording of the rules, if any model held a weapon that was NOT paired with one of these weapons, then they would never gain the +1 attack. This is most notable with Marneus Calgar, who was given a Power Sword specifically so that his Gauntlets of Ultramar wouldn't always nerf his impressive Initiative status...but this meant that he never got the +1 attack for having two Power Fists (which was explicitly a pair of weapons). Surprisingly, GW never clarified this during 5th edition's run and instead, in 6th edition, such weapons instead received the "Specialist Weapon" classification, now clearly stating that you gained +1 attack for either using any 2 weapons without "Specialist Weapon", or using two weapons (of any kind) with "Specialist Weapon".
The 8th Edition had to quickly patch out an easily-abusable loophole in its new "keywords" system. Many characters or other hero units have "aura" abilities that grant bonuses to units within a range that have a certain keyword, for example Marneus Calgar boosts all "Ultramarines" near him. Most armies have a "fill-in-the-blanks" keyword on all their units, like <Chapter> for Space Marines or <Hive Fleet> for Tyranids. The intent for these was to allow players to create their own Chapters/Hive Fleets/Guard Regiments/Etc, but if taken as Exact Words you could give factions abilities they should never have access to by giving your Sailor Earth the same name as a canon faction. Errata was printed to prevent this from working, with the website stating: "even if you named your Tyranid Hive Fleet “Ultramarines” the only stirring emotions they’d experience in the presence of Marneus Calgar would be hunger".
The first Chapter Approved book added a rule that aircraft cannot hold objectives.
In their Lord of the Rings game, one version of Frodo had an "inspiring presence" rule which stated "Frodo counts as a banner in all respects." A later reprint added "with the exception that he cannot be picked up and wielded by another model." Guess what particularly "creative" players tried to do with Frodobefore this?
In GURPS, it is possible to enchant a pair of permanent Gate spells and then arrange them to create a perpetual motion machine using electromagnetic principles that could then be tapped for an unending mana supply. However, due to the various components required, this would need a setting where both modern science existed, magic existed, and the Draw Power spell from GURPS Grimoire 3e specifically existed. In the one GURPS setting where this is canonical (GURPS Technomancer), there is an entire sidebar devoted to explaining how it specifically does not exist. This probably had something to do with the fact that David R. Pulver, the writer of Technomancerparticipated/lurked in a Usenet thread where the "Infinite Mana Well" construct was first proposed... at the exact same time Technomancer was in final playtest.
One issue of Pyramid pointed out that based on the Size/Range rules a person sitting in an open field on a cloudless day only had a 50% chance of locating the position of the Sun. This was patched in 4e Low-Tech which introduced bonuses for seeing an object it it glowed (officially for designing flares and signal fires).
In The Trillion Credit Challenge (using Traveller), contestants had to purchase and field a fleet of ships to do battle with other fleets. Doug Lenat fed the parameters of the tournament into a computer (in 1981) which suggested that instead of sending in a balanced fleet of carriers, battleships, cruisers, and so on, he should instead build thousands of tiny patrol boats. He won in a rout - though he took incredible losses, he overwhelmed his opponents through sheer numbers. The organizers then made their first patch: they added 'fleet agility' as a parameter for the following year's tournament. When Lenat entered again, his computer used much the same strategy with one change - whenever any of his ships was damaged, they would sink themselves, which kept the average mobility of the fleet up. The organizers then made their second patch - tell Lenat that it was weird to have his unorthodox plans keep winning (since, after all, they relied on ordering thousands of men to suicidenote what does he think this is, Ender's Game?) and that if he continued to enter, they would stop holding the tournament. Lenat then bowed out gracefully.
The rules for creating abominations in Old World of Darkness. If you attempt to turn a werewolf into a vampire, the werewolf gets a skill roll. He wins, he dies peacefully. He loses, he dies horribly but his soul is free. He botches, he becomes an abomination, essentially a walking Game-Breaker balanced out by crippling depression. Since there are all sorts of abilities in tWoD that can cause a skill roll to fail or critically fail, the editors in Revised Edition state that nothing short of divine intervention can affect the roll (except spending a Willpower point for an automatic success).
So many in Exalted, probably because every single character carries a Game-Breaker power... to give an example: Accuracy Without Distance, an advanced Solar Archery charm, basically says, "you can't miss". In the same paragraph where it says so, it also says "you can't shoot through the gap in someone's armor". You can almost hear First And Forsaken Lion laughing at you.
Starfleet Battles had immense problems with this in editions prior to the current edition, partly because the designers would allow rule changes if enough people wanted them or a strong enough case was made for them. It reached proportions where the 3rd edition was becoming unplayable. The current edition has an explicit decision that no such changes will be made unless they are genuine loophole closures or error corrections. The famous volume of the rules also is intended to include all situations that could arise in terms of rules interacting with each other explicitly and consistently without adding additional patches later. The difficulty of making ship design rules work properly (prior editions had more changes to these than any other part of the system) was the official reason why they were not released for a long time.
The Shadowrun supplement Court of Shadows allows the character to explore the magical inner world of the setting. But in a world with no technology, what happens to the people playing the Decker (hacker) and the Rigger (vehicle pilot)? Just to allow for this, the setting introduces enchanted mortals who have been enslaved to act as information storage devices for the Decker to interact with. If that sounds silly, the Rigger magically becomes The Beastmaster and can rig and remotely pilot, for example, a camel. And if he does, the supplement instructs the GM to give the camel the stats of a Harley-Davidson motorcycle; the bizarreness this could cause is oddly never mentioned again.
Hero Clix had to apply two, relatively quickly, after the game was launched. The original rules let flying characters carry a friendly character as they moved, with no restrictions. "Taxis", cheap pieces with no combat utility but valued simply because they could fly, rapidly became a necessary component of any team, since the mobility they granted to their team meant any team without taxis simply could not compete. WizKids double-patched it, declaring that fliers cannot carry other fliers, and that carried characters could take no further actions on the turn they were carried.
The X-wing miniatures game has constantly been doing this through its life cycles, the most (in)famous being the nerfing of Manaroo's ability to trade tokens to any friendly ship down to just friendly ships within one range band of her.