Obvious Rule Patch / Yu-Gi-Oh! Card Game

• Lists in game started as just the Limited List: normally, you can have up to three of any one card in a deck, but for game balance reasons the Limited List mandates that only one (Limited) or two (Semi-Limited) copies of certain cards can be included in a deck. Before long, players were discovering interesting ways to break the game using card combos the game designers hadn't foreseen, resulting in absurdly powerful decks that could force a win in a single turn (or even the first turn). Thus the Limited List was expanded to include Forbidden Cards, which cannot be included in a deck at all. The list is changed roughly every six months, with cards being both added to and sometimes removed from it. Even then, on occasions where a Deck outright dominates a tournament scene, the "emergency banlist", not restricted by such a schedule, can be enacted to knock the dominant Deck down a notch.
• The early history of the OCG, back in early 1999, is somewhat obscure, but when the game had just begun, there was no concept of Tribute Summoning, and each player could only activate 1 Spell or Trap Card per turn. Tribute Summoning was introduced because otherwise every game would be decided by who got to draw their Blue-Eyes White Dragon first (and this was later incorporated into the manga for the Battle City rules), while the S/T ruling was phased out entirely.
• In an interesting take on this, the formerly-Limited card "Twin-Headed Behemoth" was taken off the list because of a ruling change: Its effect (which lets it revive itself from the Graveyard at the end of the turn it's destroyed from the field with 1000 ATK and DEF) specifically states it can only be activated "once per duel". It was put at 1 after it was pointed out that multiple copies of the card would make it impossible to keep track of which copy had used its effect and which haven't, meaning anyone could abuse the confusion and reuse the card's effect illegally. Now, though, the card's ruling has changed so that only 1 copy of it owned by a player can activate its effect per duel, period. The rule change was completely arbitrary, only allowing the card to become unlimited without interfering with the reasons it was limited in the first place.
• Green Baboon, Defender of the Forest, is another monster with an unusual story: its effect allows the controller to pay 1000 LP to summon it from your hand or Graveyard if a Beast-Type monster you control is destroyed, but too many duelists were exploiting that effect by bringing it out by purposely suiciding their Beasts in battle. As such, Konami arbitrarily decided its effect can't go off if the Beast monster was destroyed by battle. This significantly weakened the monster's power and caused many players to declare They Changed It, Now It Sucks!, but there's a twist to the tale. Shortly after the ruling was implemented, guess what came out? A new monster, with the exact same stats as the original Baboon (and even named Yellow Baboon, Archer of the Forest), and the ability to summon itself from the hand if a Beast you control is destroyed by battle (this time requiring the controller to banish two Beast-Type monsters from their Graveyard to do so, instead of paying LP).
• The "Archfiend" cards, an issue resulting from bowdlerization of card names. In the Japanese version, several cards used the word "Demon" in their names, and this word was changed into a bunch of different words in the initial American releases: "Demon's Summon" became "Summoned Skull", "Demon's Axe" became "Axe of Despair", and so on. This worked fine until a series of cards that specifically dealt with cards with "Demon" in their names started to come out, so a ruling had to be issued to declare "Archfiend" as a "special category of card" which included all the cards that had "Demon" in the Japanese name. From then on, "Demon" would always be translated as "Archfiend". Later on, new printings of the Archfiend cards would include rules text stating that they were treated as Archfiends at all times, even on the Normal Monsters.
• In general, the TCG is prone to having cards be renamed in later releases, when they are incorporated into later-released archetypes and there's no easy way for the English card text to cover them. Other than the Archfiends, probably the most prominent example of this is the capitalization of "HERO" in groups of cards such as the "Elemental HERO" and "Destiny HERO" series, due to the introduction of new sub-archetypes of HEROes that weren't foreseen by the creators (and the existence of other cards, not connected to these archetypes, with "Hero" in the name).
• The "Guardian" Archetype was another nightmare when it was first introduced. This caused the one card that specifically designates a "Guardian" card to include a long list of cards that it does not apply to because they do not have "Guardian" in their names in Japanese.
• Cards have been retroactively given new names because of an archetype released years after it was first introduced, or because their current name conflicted with a new Archetype that they didn't belong in. A prime example is Sky Scout, formerly known as "Harpie's Brother". It was never intended to be a Harpie monster (its Japanese name is "Birdman"), but at the time it didn't matter too much since all the cards that worked with Harpies specifically only worked with "Harpie Lady". Later, more Harpie cards came out with a wider range of support, so to save card space from having to specify that "Harpie's Brother" didn't count, the card was renamed.
• With the release of Xyz Monsters, there was a brief period where there were very few written rules about how they actually work - one key problem was the fact that the monster used for Xyz Summoning visibly stayed on the field until "detached" by an effect. Fine, but when do effects that specify when a monster "leaves the field" trigger? Kevin Tewart, head of development for the overseas TCG, said Xyz Materials were treated as being on the field, meaning "leaves the field" and "sent from the field to the Graveyard" effects would activate upon being detached, and all hell broke loose. Two already powerful cards, "Tour Guide of the Underworld"^note Summoned "Sangan" from the Deck, then used both to Xyz Summon "Number 17: Leviathan Dragon" and use its effect to immediately detach "Sangan" to activate its effect and "Reborn Tengu"^note replaces itself with a copy from the Deck whenever it leaves the field got so absurdly broken that a single copy could easily fetch well over 100 dollars. Konami quickly made a rule change to have the TCG rules coincide with the Japanese OCG rules: Xyz Materials are not treated as if they were on the field, but effects that activate when they "leave the field" do not trigger when a monster becomes an Xyz Material. (This also meant that cards that activated when sent from the field to the Graveyard couldn't activate their effects when detached, since they wouldn't be on the field by the time they were sent to the Graveyard.) It's just as weird as it sounds.
• "Light and Darkness Dragon" used to have so much text on it that Upper Deck/Konami never fit a very important sentence on it: its negation effect can only activate once per chain. If Konami had never announced that ruling, Light and Darkness Dragon would continually keep activating to negate ITSELF until its attack and defense got so low that it couldn't activate any more. This ruling also gives Light and Darkness Dragon its main weakness, which is that a card can be chained to its effect, which it will be unable to negate.
• In early 2015, a card-specific rule patch was introduced: "Dark Strike Fighter", a card banned mere months following release given its ease of summoning, high attack, and powerful burn effect, had the latter effect limited to Main Phase 1, and only once per turn, period. Following this change to Dark Strike Fighter, Konami announced a new stance regarding forbidden cards. Now, every so often they would introduce several errata by changing card text to one or multiple cards on the Forbidden List. Notable examples are Crush Card Virus and Ring of Destruction. The overriding sentiment is, it's better to make a card weaker and actually usable than keep them as strong as they are originally and have them remain on the list forever.
• Pole Position was infamous for preventing all sorts of actions due to Upper Deck Entertainment rulings on infinite loops resulting in a Logic Bomb. In short, neither player may play a card that would trigger an infinite loop with no changes to the game state, but if they're forced into such a situation (such as using Bait Doll to force Pole Position's activation, or attacking a face-down monster in certain situations) then the card that caused the loop would be destroyed. When Konami gained control of the TCG, they didn't make any rulings that would override the UDE rulings, so players continued to use them to determine card interactions with Pole Position. Years later, a deck known as "Pole Position FTK" began to gain notoriety for abusing these old rulings to lock the opponent out of playing any card to the field.^note This was done by summoning a card that gained ATK based on the number of cards an opponent controlled, like Invoked Purgatrio, and a slightly stronger monster equipped with an Equip Spell that boosted its ATK. With Pole Position active, your opponent placing a card on the field would cause the former monster to have ATK higher than the latter's original ATK, triggering an infinite loop, and thus would be an illegal move. As such your opponent would be unable to play most cards unless it was a monster stronger than both monsters, or using a Kaiju to remove one of the two monsters. It didn't have an impact on the metagame, being mostly used to troll, but Konami would eventually update their tournament policy to allow players to willingly trigger infinite loops, at which point the offending card would be sent to the Graveyard.
• A German player, Maik S., entered the 2007 German National Tournament with a 2,222-card Main Deck and 222 card Fusion Deck, as part of a joke with a friend. Maik, himself an official judge for Upper Deck Entertainment (the company running the tournaments), knew there was only a minimum deck size limit for the main deck or the Extra Deck (then known as the Fusion Deck), without a maximum. The deck used was practically impossible to shuffle effectively, could not have easily verified contents, and was housed in a holster that could be seen from several tables away. After the first duel, the event's chief judges convinced Maik to drop out of the tournament. In less than 1 year, the first ever standardized "Master Rules" format introduced a restriction on deck sizes to 60 cards, and a restriction on Fusion (Extra) Deck size to 15 cards, limitations that have stayed in place to this day. This occurrence at a national event may have led to this rule being put in place.
  • The deck, which was practically impossible to use effectively, included effects that would require the deck to be shuffled afterward. As this would take a very long time to be shuffled, judges ruled him out of the tournament as it could cause games to take a very long time and thereby hold up the entire tournament. It was also highly inconvenient for judges, because in the event that they needed to do a deck check (which would require them to inspect each card in the deck and compare it against the deck list the player provided, which in this case was several pages long), it would, again, take a very long time. Maik would go on to be an even more influential judge, authoring some of the clarifications and rulings that addressed key holes in the game. Some of these rules are used to this day.
• "Once per turn" restrictions were as written — you could use the monster's effect once per turn while it stayed on the field. However, should that monster leave the field and return, or if you got another copy of that monster onto the field, you can use that effect an additional time. Clever use of Summoning loops led to players getting way more mileage out of these effects than the designers intended. Now, you're more likely to see the phrase "You can only use the effect of (card name) once per turn", closing the loophole for most effects, especially the more potent ones. Some formerly banned cards even have this modern clause in their new text to rein in their power enough for them to come off the list.
• Most of the new Monarchs Spell and Trap support require that the player have no cards in their Extra Deck to trigger their effects. One of the most prominent among them is Domain of the True Monarchs where it can lock out the opponent of special summoning their Extra Deck in addition to the player controlling a Tribute Summoned monster. However Monarch players tend to deceive their opponents by placing cards in the intended Extra Deck before revealing that it isn't actually part of the deck which led to ruling hassles and declarations of unsportsmanlike conduct. Shortly after Emperor of Darkness Structure Deck was released in the TCG, a player's Extra Deck is now public knowledge even before the duel has started and players must truthfully respond whether or not they have any cards in the Extra Deck if asked.
• Firewall Dragon was one of the most accessible Link monsters with no restriction on what kinds of monsters you can use to make it, which led to Token-generating effects being very effective at Summoning it. Several Token-making cards got banned as a result. Subsequent Link monsters would be more stringent with their requirements — sometimes requiring Effect Monsters, sometimes requiring monsters with different names, and even outright excluding Tokens from their Summon restriction — just to deter using Tokens as material.
• Cyber Blader has an effect that negates all of the opponent's cards if they control exactly 3 monsters, but this led to bizarre infinite loop situations in cases where the third monster was one they had taken control of with an Equip Card like Mark of the Rose.^note Cyber Blader would negate the Equip Card, causing control of the equipped monster to return to its owner, but that meant the opponent no longer had 3 monsters, so the Equip Card wouldn't be negated anymore, so it'd take control of the monster and cause them to have 3 monsters again, repeating the process. Konami eventually released an errata to make it only negate an opponent's activated effects, meaning continuous effects such as Equip Cards would work as normal, preventing it from creating loops.
• In Yu-Gi-Oh! Master Duel, one of the first event hosted in the game was an Xyz festival that forced players to use only Xyz monsters, banning not just non-Xyz Extra deck monsters but also archetypes that don't revolve around Extra deck plays such as Eldlich and Monarchs to prevent players from cheesing the event. To encourage players to participate in the event, players are awarded points even if they lost the duel (100 for winning & 50 for losing and none at all if they surrender). However, players soon found out that it was far more efficient to lose twice in quick succession than to play a legitimate game that could last up to 15 minutes, thus this caused most of them to run self-burn decks that were designed to quickly deplete their own LP as fast as possible in order to attain gems as efficiently as possible, with only little investment required compared to the cost of building a competent Xyz deck. As a result, Konami increased the reward for winning from 100 to 500 in order to incentivize legitimate Xyz duels. Subsequent events such as N/R and Synchro Festivals flat-out bans any cards that purposely deplete your LP as fast as possible while still keeping the greater reward for winning in order to deter any further self-burn decks in the events.