Not The Intended Use / Yu-Gi-Oh! Card Game

• There are quite a few cards that operate at the cost of letting your opponent draw cards. Since you instantly lose if your deck runs dry some savvy players built decks entirely out of these cards, using their intended effects as defense while they bled their opponent of all his resources.
• 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. It was even released in the same set as Ring of Destruction, a card that does massive amounts of damage to you and your opponent; the Ring/Barrel combo swiftly became one of the most popular in the game.
• Gate Guardian is a high-ATK monster that's incredibly hard to get out... but you don't need to get him on the field to fuse him with a card released much later, UFOroid, to form an even stronger UFOroid Fighter, or discard him to pump up the ATK of Zubaba General for a monster with nearly 6000 ATK.
• Royal Oppression was designed to be used as an anti-meta card to cripple meta decks reliant on special summoning. But then players began to realize that they could swarm the field with big monsters and then activate it during the opponent's turn to lock them out from their own special summons. There's also a surprising number of cards where Royal Oppression either cannot stop the summon for timing reasons (Tragoedia), or does nothing to stop the summon due to the card being able to "try again" (Vayu the Emblem of Honor), so comboing them with Royal Oppression made it even more one-sided. This ultimately resulted in it getting banned. Other Continuous Trap "floodgate" cards like Vanity's Emptiness and Imperial Order were also abused in the same manner and thus banned.
• Divine Sword - Phoenix Blade is a subpar Equip Card, exclusive to Warriors, that provides a miniscule ATK boost. However, it was also a staple in one of the most popular Decks of its day, Diamond Dude Turbo, for its secondary effect of "banish two Warriors in your Graveyard to add this card back to your hand." Players would use Phoenix Blade's secondary effect over and over, use it as a discard fodder, and then activate Dimension Fusion to resummon all the cards they banished. Additionally, years later, players realized that since it could keep returning to the hand, as long as you had Warriors in your Graveyard, Phoenix Blade was effectively a replenishable source of discard fodder—making it very useful in decks focused around the Knightmare lineup, which all have discard effects and frequently saw use in Warrior-heavy decks like Gouki and Dark Warrior, and could be gotten into the Graveyard immediately via Isolde. The card was actually banned in the OCG for that reason.
• Which players are the biggest users of Toon Table of Contents, a card that lets you search out any card with "Toon" in its name? If you guessed Toon players, you'd be wrong - Toon Table of Contents is used in nearly all modern Exodia decks. This is because you can use Toon Table of Contents to search out copies of itself, and there's no limit to how often it can be used. That translates to three cards removed from your Deck, and three Spell Counters on Royal Magical Library, and you can then either search out Toon World for a fourth counter, or Blue-Eyes Toon Dragon to discard for Trade-In. Toon Table of Contents is actually straight-up better than Gather Your Mind, a card where this use is the intended use. Sky Striker decks, which rely on having at least 3 Spell Cards in the Graveyard to make full use of their Spell Cards, also tech in copies of Toon Table of Contents as a quick and easy way to get those requisite Spells into the Graveyard.
• The intention of Exodius the Ultimate Forbidden Lord was to be a sort of backup win condition in Exodia decks, or possibly a deck in itself. It summons itself by shuffling all monsters from the Graveyard into the Deck, gains 1000 ATK for every Normal Monster in your graveyard, mills a monster each time it attacks, and wins the game automatically after having milled five Exodia pieces by that effect. Unfortunately for Exodius, playing it as intended happens to be hideously impractical, as it requires you to make five different attacks with a monster that needs to make two before its stats become even tolerable, and it has no protection whatsoever—even in its day, it'd be lucky to make one attack, much less five. Years later, though, when Xyz and Links were added, players realized that Exodius was on the short list of generic monsters that could summon itself from the hand entirely regardless of board state (and one of the very few to be level 10), and its condition of shuffling your Graveyard monsters back into the Deck, though intended as a downside, could actually be helpful for recycling purposes. This resulted in a lot of players running it in decks like Trains, Hieratics, and especially World Chalice, that used no other Exodia cards and made no attempt to use any of its effects outside of the summoning condition.
• The intended playstyle of the T.G. archetype was clearly supposed to be based on spamming out powerful Synchro monsters, through using the replenishing effects of their Main Deck monsters to build up a large amount of resources that could be used for Synchro Summoning. However, a number of players realized that those same resources could be used to simply beat the opponent normally, and so T.G. became instead best known as a stun deck, focusing on using a massive pile of Traps to interrupt the opponent's plays while an endlessly-replenishing horde of monsters beat them up.
• This is what got Self-Destruct Button (a card that forces the Duel to a draw if your opponent has 7000 more LP than you) banned. Instead of using it as a last-resort Taking You with Me, players would fill their decks with cards that gave your opponent LP, then activate Self-Destruct Button at the first possible opportunity. It may seem meaningless at first glance, but in tournament settings, players would win game 1 off Final Countdown and then force ties for the next two to walk away with a win for the match.
• Last Turn is meant to be an extreme last-resort gamble: usable only when you're at 1000 or lower LP, it picks a monster on your side of the field, then sends everything else to the Graveyard, then your opponent Summons something from their Deck and attacks, with the survivor winning the Duel. The gamble involved was the risk that your opponent could Summon something stronger. However, Last Turn ignored the possibility of cards that could stop the Special Summons in some manner (Jowgen the Spiritualist, Vanity's Fiend, Last Warrior from Another Planet) and the existence of cards that could pay massive amounts of LP (Wall of Revealing Light, Inspection, Backs to the Wall). If the opponent couldn't Summon a monster for Last Turn, you won by default. The resulting "Last Turn OTK" sent Last Turn to the banlist.
• Lyrilusc -- Independent Nightingale is intended to be Magikarp Power — its effects are dependent on its level, and its level is dependent on the monsters used to summon it. If you work your butt off, you can fuse together the proper monsters to make its level 5 or 6... or you could just summon it with Instant Fusion and Tribute it for The Tyrant Neptune, which absorbs the effects of its Tributes and is Level 10 normally. For just two cards and 1000 LP, you could have a 6000-ATK monster that's unaffected by card effects and can inflict 5000 damage every turn. The combo was deadly enough that Tyrant Neptune was banned before Nightingale was even released in America.
• Similar to the above, Supreme King Dragon Starving Venom has the same effect as The Tyrant Neptune. But while it's two Levels lower, you can bring it out with two DARK Pendulum monsters and no other cards, meaning you don't have to hope you draw into it like you would with Neptune. It also packs 6800 ATK, making it more powerful than Neptune overall.
  • Supreme King Dragon Starving Venom could be abused in other ways, namely through Odd-Eyes Revolution Dragon. Revolution Dragon has an effect that makes it gain ATK/DEF equal to half your opponent's Life Points - which would give it 4000 ATK at the start of a duel. You could also use it to shuffle all other cards on the field and in Graveyards into the deck, which would seriously mangle most decks and leave your opponent open to an attack. Unfortunately, its harsh summoning condition makes it not worth bringing out. Its main use was its effect, which let you pay 500 LP to search out Odd-Eyes Pendulum Dragon. Starving Venom ties this all together: you drop Revolution Dragon into the Graveyard with its discard effect, bring out Starving Venom, nuke the field, and attack with a 6800 ATK monster. Your opponent most likely won't have the resources to come back from the duel at this point, much less have a way to take down this monster.
• This hit no less than two of the main Guardian equips, originally designed to provide small boosts and make summoning their Guardians possible. Butterfly Dagger -- Elma returns itself to the hand when destroyed, with the intent being to keep the opponent from destroying it. As it turned out, it also comboed with Gearfried the Iron Knight, which destroys anything equipped to it, and could therefore be looped infinitely in combination with cards like Royal Magical Library or Magical Marionette to build up infinite Spell Counters. Rod of Silence - Kay'est has the secondary effect of destroying Spells that target the equipped monster, obviously meant for protection. Instead, people ran it in Vylons, where thanks to Vylon Element, which summons a Vylon Tuner whenever one of their equips is destroyed, and the standard Vylon equips, which search out more of themselves when destroyed, they could rapidly swarm the field with Tuners and usually summon Omega on the first turn.
• The Destiny Hero archetype was intended to be played as its own set of cards, with a focus on Standby Phase effects, stalling, and building up tribute fodder for powerful boss monsters. Instead, many players realized that they worked much better as a draw engine, combining the effects of Destiny Draw, Malicious, Diamond Dude, and Disk Commander, along with the search power of Elemental Hero Stratos to shore up on their own strategies. This particular case was big enough, that the Destiny Hero engine in its various forms have been the core strategy of multiple decks that is considered a Tier 0 Game-Breaker.
• A particular breakout example in the competitive scene is the Gem-Knight archetype's own Brilliant Fusion, a Continuous Spell that can Fusion Summon a Gem-Knight Fusion monster by sending the corresponding material from the main deck. Intended as a Gem Knight support, it gained notoriety for its ability to Fusion Summon Gem-Knight Seraphinite by sending a Gem-Knight and a Light monster from the deck. This combo gives you an extra normal summon, puts a monster on the field as a fodder for other summons, and mills 1 Light attribute monster of choice to set up further combos, while also thinning the deck by 2. Brilliant Fusion is so popular as a deck engine that at one point, Gem-Knight Garnet, a completely ordinary 1900 vanilla, fetched quite a price on the secondary market entirely because it was the highest ATK Normal-summonable Gem-Knight monster. The Brilliant Fusion engine even birthed a new term amongst the fanbase: a "Garnet" is a card that is a mandatory engine requirement, yet is completely useless while in the hand.
• Wetlands is a Field Spell that gives a fairly hefty 1200 ATK boost to level 2 or lower Aqua/Water-type monsters. Given its time of release and theming, it's not hard to assume it was meant to be used in Frog decks, which generally fit the bill and have very poor stats, meaning they'd appreciate the boost. However, Frog statlines are so poor all-around that they couldn't really make use of it (only Swap Frog and the rarely-used Submarine Frog have tolerable beater stats with it active), and preferred making use of other engines such as Tributes or Xyz when they wanted more firepower. Instead, it found its home in Ice Barrier decks (which frequently used it to power up Prior, Dewdark, and Cryomancer and bolster their lacking offense) and the rather silly Slime Token Beatdown (which comboed it with Token Stampede to churn out 2700 beaters).
• Probably the silliest case of "archetype card that nobody uses in its archetype" is M-X-Saber Invoker. It was designed to try and ease X-Sabers, normally a powerhouse Synchro deck, into the Xyz era. To that end, it was a Rank 3 with the ability to Summon a Level 4 EARTH Warrior or Beast-Warrior from the deck, seemingly meant to support the swarm-happy playstyle that X-Sabers were known for. However, X-Saber players looked at their roster and realized that only four X-Sabers fit the criteria, only one of them was actually good, and that one couldn't use its effect if summoned by Invoker, so they mostly ignored it. As it turned out, though, a lot of Decks could make Invoker easily and had a monster that did meet the qualifications and they wanted out quickly, including Wind-Ups, Madolches, Six Samurai, Zoodiacs, Goukis, and even certain FTK builds. Invoker actually got banned in the TCG because pretty much any deck that wasn't its own archetype could get some mileage out of it.
• Goukis themselves, based on their monster effects and their Links, were intended to be played as an aggressive beatdown deck that would use its main gimmick (all main-deck Goukis search out another Gouki upon being sent to the Graveyard) as a way to replenish its resources and win grind games. Instead, players realized that when combined with the pre-errata Firewall Dragon (which could summon monsters from the hand upon a monster being sent to the Graveyard), Goukis were able to keep putting monsters on the field for use as material, turning the archetype into a very easy swarm deck that flooded the field with Link monsters and could lock the opponent out of summons altogether. This was a major factor in Firewall Dragon's banning and later errata, making it only able to summon Cyberse-type monsters and therefore removing its ability to spam Goukis (which are all Warriors).
• Rokkets, another VRAINS archetype, were given a firearm theme where Rokket monsters could destroy themselves to take out the opponent's cards when targeted by a Link Monster's effect and then Special Summon another Rokket from your deck at the end of the turn where they get destroyed, intending a playstyle of pulling out a boss monster like Borreload Dragon and using its effect to fire off Rokket monsters as ammo, which would then reload your field with more Rokkets. In practice, the deck's swarming tools meant that their gimmick was foregone in favor of being used as an engine in Dragon Link decks, with the Rokkets being little more than Link Material for more generic monsters.
• The Lunalight archetype was clearly supposed to be based around Fusion Summoning, considering that all of their Extra Deck monsters are Fusions. However, at the height of their competitive viability, the ease at which the deck could swarm the field with Level 4 monsters made them far better suited for Xyz Summoning.
• Crystal Beasts are built around gathering their (relatively weak) monsters in the field or Graveyard in order to summon their boss monster Rainbow Dragon. However, because Rainbow Dragon is so clunky and demanding to actually summon even in a dedicated deck, they became used for Xyz and later Link spamming instead (in spite of the archetype not having any Extra Deck cards until Rainbow Dragon was given a retrain) since a strong Extra Deck toolbox was far more valuable than a gimmicky ace monster. Even back around the time of their release, their effect to put themselves in your Spell and Trap Zone as a Continuous Spell when destroyed, which was ostensibly used for fueling effects and maintaining card advantage, instead made them excellent fodder for Hamon, Lord of Striking Thunder, which also had the same ATK as Rainbow Dragon while being less costly to summon.
• Union Carrier is, as the name suggests, intended to be support for union monsters, particularly the A-to-Z archetype, although its restrictions are also loose enough to support other Equipping archetypes like Inzektor, Dragunity, and Cyberdarks. In practice, Union Carrier's equipping restrictions were too loose and it was used for anything but those kinds of decks — it's been used to equip Dragon Buster Destruction Sword to any DARK or Dragon monster (instead of Buster Blader as the card was intended for) to create a lock, or to easily force the "leaves the field" trigger of Aslla Piscu for FTK strategies. Union Carrier was eventually banned for creating interactions that old cards were not clearly designed for, and going forward, "leave the field" triggers tend to specify "leaving from a Monster Zone" to avoid cards being broken in a similar fashion.
• Predaplant Verte Anaconda. Despite being part of the Predaplant archetype, it saw way more usage in literally every other deck due to how easy it was to summon (2 effect monsters). It's effect let it basically search fusion Spells from the deck, meaning that it could easily bring out juggernauts such as Red-Eyes Dark Dragoon and Destiny HERO - Destroyer Phoenix Enforcer in a few steps thanks to the existence of fusion Spells that could themselves fuse monsters from the Deck. The overusage was so wild that it was banned in both formats.
• Branded Expulsion trades one of your Fusion Monsters to put a monster from your GY on each side of the field (or both on your field if you sacrificed an Albaz Fusion). If used alongside Fallen of Albaz you can use the monsters you Summoned alongside his effect to set up another Albaz Fusion even during the opponent's turn. However, it's more commonly used to plant detrimental monsters like Ra's Disciple and Ido to lock the opponent out of Summoning anything. Setting this combo up is especially easy given the flexibility of Albaz Fusion Materials and the ability to Fusion Summon using cards from your Deck.
• Pre-errata Future Fusion was a card that would Fusion Summon a monster in two turns at the "cost" of dumping the materials from the Deck to the Graveyard. Since two turns is a long wait in Yu-Gi-Oh, even during the GX era where this was released, the real value of the card was the massive graveyard setup it could enable when applied to cards with more generic or wide-ranging materials. Just for example, picking Chimeratech Overdragon would make the card Painful Choice on steroids since there's no limit to how many Machines one could dump, and to drive the point home, this conflicts with the intended use since Chimeratech would self-destruct if summoned that way. This was why it was banned and later nerfed to add a one turn delay to the Graveyard dumping effect.
• The Vendread archetype is a series of Zombies built around Ritual Summoning, with many of their monsters being able to summon themselves from the Graveyard and giving Vendread Ritual Monsters additional effects if Tributed from the field for a Ritual Summon. However, most top-performing Vendread decks ran no Ritual cards whatsoever; between generic Zombie support cards and their own effects allowing them to quickly flood the board, they instead proved to be excellent at spamming generic Link Monsters, which was far less clunky and allowed access to far better monsters than attempting to play them as intended.