Unwinnable by Mistake/Tabletop Games/Card Games

Yu-Gi-Oh!
"Kaiba: Yugi, you took advantage of a glaring flaw in the Duel Monsters rulebook. Truely, you are an honorable duelist."
 * There's an old strategy in the Yu-Gi-Oh Card Game known as the Lockdown, a deck that forces a continuous loop that -- once in place -- makes it impossible for the opponent to counter:
 * Combining any Tuner on the field, Imperial Iron Wall, Cannon Soldier, and Quillbolt Hedgehog makes an infinite burn loop that ends in an OTK. Considering how the game goes, a player can easily leave a destruction card in their hand for later, only to be met with this happening as soon as the second turn. This can be pulled off more easily if you have Dark Verger, which eliminates the need for Imperial Iron Wall but requires a Plant-type Tuner.
 * There are many other OTKs involving Cannon Soldier or Mass Driver. More are discovered all the time. A current popular OTK or FTK is the Frog FTK revolving around Substitoad, Ronintoadin, Swap Frog, and Mass Driver.
 * The first popular lockdown combo was the Yata-Lock combination. The player needed to have a Sangan or Witch of the Black Forest on their side of the field, and have one LIGHT and DARK monster in their Graveyard. They remove the two cards to Special Summon Chaos Emperor Dragon and then pay 1000 Life Points to nuke the field and players' hands. The effect of Sangan or Witch would be activated; they would be sent to the player's Graveyard, and the player could search their deck for Yata-Garasu and add it to their hand. Then they could play it and attack their opponent with it. It only did a tiny bit of damage, but its effect prevented your opponent from drawing a card on their next turn, which left them defenseless. When the Ban List was first released, these cards were quickly placed on it.
 * Yata-Lock worked before Invasion of Chaos; that particular combo just ensures that it works. If your opponent has no way of stopping Yata-Garasu, it attacking means that they cannot get a way of stopping it, resulting in a Death By a Thousand Cuts.
 * There are a few lockdowns intentionally made for the game, such as the earlier Tornado Wall card. But most intentional lockdowns only affected one aspect of the game or had an upkeep that would eventually kill the player for using it too long. Almost all of the other lockdowns were created when players used the cards outside their intended purpose.
 * Amusingly, Yugi uses a loop to win in the anime, so it indeed seems a legal strategy. Naturally, Yu-Gi-Oh the Abridged Series has fun with this.


 * If the last card in your hand and field happens to be Sword Of Deep-Seated or Horn Of The Unicorn or a card with the same effect (both have the effect of being placed at the top of your deck if they're sent to the graveyard) you'd better pray that you don't get attacked by White Magical Hat or Mefist The Infernal General or a monster with a similar effect (both have the effect of randomly discarding one card from your opponent's deck). You would end up in an endless loop of discarding and re-drawing the same card over and over with no way to counter.
 * There is only one way of getting out of this. You have to put that card on the field. You can equip it to your opponent, or better yet just set it. It's an Instant Win Condition in the video game versions because the AI is too stupid to get itself out of this situation.
 * There are also combos which force your opponent into an unwinnable position. If you played Last Turn and chose a monster that forbids the opponent from special summoning, you instantly win because the opponent would have no other monster on their side of the field. This was likely never intended in the card's design.
 * The Last Turn card was meant to be a duel between two Monsters. The Special Summoning part was to simplify the rules so that they didn't create a new mechanic just for this one card. But a literal interpretation of the rules made it unwinnable for whoever isn't in control of Jowgan the Spiritualist, the monster who forbids special summoning after he is summoned.
 * There are also loops that never resolve. The biggest offender is Pole Position. If two monsters on the field had similar (but not the same) ATK, and the weaker one became the stronger through a spell card, then Pole Position would continually activate. It makes the strongest monster immune to spell cards and takes away the ATK boost -- but now the monster is only the second strongest. So Pole Position would shift to the other monster -- and then back to the first one when the spell card kicks in again... ad infinitum. They had to make a new rule: in such a situation, you are not allowed to activate the offending card, even if you normally could.
 * Most loops which can never willingly be stopped are not allowed to be activated intentionally at tournaments.
 * There is also a decktype gaining popularity that runs on this. Called "Empty Jar", it's a deck that forces your opponent to throw away his entire deck in one turn. A major complaint about this deck is that it kills you and you won't even have a chance to play a single card or even take your turn. With that said, pray that the player running this deck doesn't go first, because all he has to show you in his hand is a Needle Worm/Morphing Jar and a Book of Taiyou, and he wins. Full stop.

Magic: The Gathering

 * There are many indefinite loops possible in this game. The rule is that if the game ends up in an unstoppable loop, then it ends in a draw; the most common of these involves Animate Dead and Worldgorger Dragon. If it is stoppable, then the players simply decide how many times the loop occurs.
 * The usual trick with Animate Dead and Worldgorger Dragon is to combine it with another effect which can take place while one of the infinite looping abilities is on the stack, usually Bazaar of Baghdad to fill up the graveyard so Animate Dead can get a 20/20 with haste and flying or something similar.
 * The rules have on occasion been changed (used as a tournament rule when a draw is not an option) so that an unbreakable loop counts as a loss for the player who created it.
 * Magic has a bunch of cards with the Nightmare creature type. The most (in)famous of these is called Faceless Butcher. What this does is that when it comes into play, it removes a creature from the game other than itself. When it leaves play, the removed creature comes back. So, to draw the game, make sure there are no creatures in play. You need three Butchers (let's call them A, B and C.) Play Butcher A. Nothing happens since there are no legal targets for his first ability. Play Butcher B. B has a legal target: A. Remove A from the game. Play Butcher C. Butcher C has a legal target: Butcher B. Now B has left play, so the second ability triggers and resolves: Return the removed creature to play. The removed creature was Butcher A. A comes into play and has a legal target for its ability: C. Remove C from the game, which bring back B, which removes A... unless someone can either counter one of the abilities (only two or so cards in the entirety of the card pool targets triggered abilities) or can kill one of the butchers before the abilities happen, you've created an infinite loop and the game is a draw.
 * The Lorwyn/Shadowmoor blocks had several creatures with the Champion ability, which removes a creature you control from the game, often with restrictions on the sort of creature it can target. This can be used to create loops.
 * Also note that such loops can lead to a game-ending condition if combined with other cards like Pandemonium.
 * These days in Magic, the O-Ring Lock is better known than Faceless Butcher, with three of Oblivion Ring. Works exactly the same way, though, so if there are no other non-land permanents, you've just locked the game.
 * Actual Lock strategies: decks that make it impossible (or almost impossible) for the opponent to win, often long before the Lock deck itself wins. There have been many decks in Magic: The Gathering that do this, such as Scepter-Chant.
 * Play a Stuffy Doll, but target yourself with its damage-sharing ability instead of an opponent. Then enchant it with Pariah, which bounces damage off of you and back on to the Doll. Then tap it to deal one damage to itself. You now have one damage being passed around an infinite number of times. If they disenchant the Pariah, you take 1 from stuffy and that's it. (The actual process is. Stuffy deals 1 damage to itself, that ability resolves (stack is empty at this point), then Stuffy's ability is triggered, dealing 1 damage to you. Then when it begins resolving, pariah redirects the damage to itself. Then it triggers again. The only time to interrupt this loop is when the 1 damage goes on the stack, in which then pariah is no longer around when the 1 damage resolves, so it will be dealt to you.
 * If you have Repercussion, boom! You now have an infinitely exponentially increasing stack of damage constantly bouncing between you and the Stuffy Doll, with no way for it to ever resolve. Unless your opponent is sitting on a Disenchant, in which case he just waits until the damage is up around 10 billion or so, and then disenchants the Pariah...
 * One could also use Furnace of Rath to achieve this same end... INFINITE ACCELERATION
 * Because of Platinum Angel (ability: you can't lose the game and your opponents can't win the game) and Abyssal Persecutor (ability: you can't win the game and your opponents can't lose the game) it is possible for two players to end up with some combination that prevents either from winning and then exhaust all possible means in either deck for the responsible cards to be destroyed.

Pokémon
"Whenever an attack (including your own) does 30 or more damage to Mr. Mime (after applying Weakness and Resistance), prevent that damage. (Any other effects of attacks still happen.)"
 * A particularly famous - albeit rare - example in the Pokemon TCG involves two primary cards to establish a perfect stalemate: Mewtwo LV.X (Legends Awakened), a Pokemon protected entirely from non-evolved Pokemon; and Uxie (Legends Awakened), a card able to return itself - and all cards attached - back to the deck via its Psychic Restore attack. So, when both players are using decks with both cards, as well as no evolved Pokemon, the game often ends perfectly tied, with no remedy per the rules in sight.
 * To make matters worse, this stalemate has no practical remedy in tournament play at all: if it happens, you're in for a long, drawn-out 40 minute round. When it's all over, the judges will either A) make you go to sudden death all over again, where this could repeat indefinitely, or B) simply give you and your opponent double game losses for delaying the event (ties are not allowed).
 * In the early game, before all of the fancy stuff, a simple locked game could be formed with both players having only a Mr. Mime on the field and nothing to cancel abilities. The problem arose with Mr. Mime's Pokepower:


 * They had only one attack, Does 10 damage plus 10 more damage for each damage counter on the Defending Pokémon, and a weakness to psychic, which back then would mean damage was doubled. Each player could attack once doing 20 damage. All subsequent attacks would deal 60 damage, more then double what the wall says it will resist. The only hope is that someone can deck the other player.

Redakai

 * Froztok: Blue Elemental costs only 5 Kairu to play, has great blue defenses, and the devastating ability which says "Your opponent can only play one attack per turn". This is used in a lockdown strategy known as "Frudge-Slap". What happens is you combine Froztok; Blue with an attack called "Sonic Slap", which can be played as a react ability to stop your opponent's one attack. Add in "Drudger: Excavation machine" which can pull a card from the graveyard once per turn, and you have an infinite loop. Only five cards have a chance of breaking the loop, and they are either extremely rare or almost impossible to use.
 * This can be set up as early as turn two. On turn one, Play Drudger onto "Ky: Stax Leader" which increases your maximum Kairu. On turn two, play "Froztok: Blue Elemental" onto Boomer, who decreases the cost of a monster played on him by one, and you have enough left over to start slapping.