Infinite loop

An infinite loop is a never-ending cycle of effects activating in response to each other, or a series of Continuous Effect(s) that never settle.

There are two distinct types of infinite loops: loops with no net change and loops with net change. There are also controlled loops, which are similar to infinite loops but are instead created by a player intentionally repeating a set of actions or making the same decisions rather than game mechanics.

Loops with no net change
These occur when a series of mandatory actions repeat after each other in a cycle, and there is no net change after each cycle, such that the game state is identical at the beginning and end of any cycle.

These types of infinite loops are illegal in gameplay, meaning that a player may not voluntarily activate any card if its activation would cause such a loop. If such a loop is caused by something involuntary, such as a player conducting their normal draw or a mandatory effect, then the card ruled to be the source of the infinite loop is automatically destroyed by game mechanics.

Pole Position
The mechanics of "Pole Position" naturally lend itself to causing such infinite loops when combined with Spell effects that directly or indirectly change a monster's ATK; see Card Rulings:Pole Position for further examples.

Voluntary
"Pole Position", "Luminous Spark", and "Gemini Elf" (1900 ATK) are face-up on the field.

If a player attempts to Summon "X-Head Cannon", the following would occur:
 * "X-Head Cannon" would be Summoned with 1800 ATK.
 * Since "X-Head Cannon" is a LIGHT monster, "Luminous Spark" increases its ATK by 500, to 2300.
 * "X-Head Cannon" is now the monster on the field with the highest ATK, so "Pole Position" makes it unaffected by Spell Cards.
 * "X-Head Cannon" is now unaffected by "Luminous Spark", reducing its ATK back to its original ATK of 1800.
 * "X-Head Cannon" is now no longer the monster on the field with the highest ATK, so it is no longer unaffected by Spell Cards.
 * "Luminous Spark" now increases the ATK of "X-Head Cannon" by 500, to 2300.
 * "X-Head Cannon" is now the monster on the field with the highest ATK, so "Pole Position" makes it unaffected by Spell Cards.
 * The above sequence would repeat infinitely, with the ATK of "X-Head Cannon" never settling.

Therefore, it is an illegal move for either player to Summon "X-Head Cannon".

Involuntary
"Pole Position", "Opticlops" (1800 ATK), and "Muka Muka" are face-up on the field. "Muka Muka" is equipped with "Axe of Despair", and its controller has no cards in their hand (making "Muka Muka's" ATK 1600).

When "Muka Muka's" controller conducts their normal draw:
 * "Muka Muka's" ATK becomes 1900.
 * "Muka Muka" is now the monster on the field with the highest ATK, so "Pole Position" makes it unaffected by Spell Cards.
 * "Muka Muka" is now unaffected by "Axe of Despair", reducing its ATK to 900.
 * "Muka Muka" is now no longer the monster on the field with the highest ATK, so it is no longer unaffected by Spell Cards.
 * "Axe of Despair" now increases the ATK of "Muka Muka" by 1000, to 1900.
 * "Muka Muka" is now the monster on the field with the highest ATK, so "Pole Position" makes it unaffected by Spell Cards.
 * The above sequence would repeat infinitely, with the ATK of "Muka Muka" never settling.

Since this loop was caused by a mandatory action, it cannot be prevented by restricting the player's actions. Since the loop cannot be avoided, "Pole Position" will be destroyed by game mechanics.

Voluntary
Player 1 controls three monsters, one of which is owned by Player 2, but being controlled by Player 1 due to "Snatch Steal".

If Player 2 attempts to Summon "Cyber Blader", the following would occur:
 * Since Player 1 controls three monsters, the effect of "Cyber Blader" negates the effect of "Snatch Steal". The 'stolen' monster returns to Player 2's control.
 * Since Player 1 now controls two monsters, the effect of "Cyber Blader" instead doubles its ATK. "Snatch Steal"'s effect is no longer negated, so Player 1 gains control of the 'stolen' monster.
 * Since Player 1 now controls three monsters, the effect of "Cyber Blader" negates the effect of "Snatch Steal". The 'stolen' monster returns to Player 2's control.
 * The above sequence would repeat infinitely, with the player who controls the monster equipped with "Snatch Steal" never settling.

Therefore, it is an illegal move for Player 2 to Summon "Cyber Blader".

If Player 2 had already controlled "Cyber Blader" before Player 1 activated "Snatch Steal", then Player 1 activating "Snatch Steal" (on a monster other than "Cyber Blader") would be an illegal move.

Involuntary
Player 1 controls two monsters, one of which is owned by Player 2, but being controlled by Player 1 due to "Snatch Steal". Player 1 also controls "Jam Breeding Machine". Player 2 controls "Cyber Blader".

During Player 1's Standby Phase, the following would occur:
 * "Jam Breeding Machine's" mandatory effect Summons a "Slime Token" to Player 1's field.
 * Since Player 1 controls three monsters, the effect of "Cyber Blader" negates the effect of "Snatch Steal". The 'stolen' monster returns to Player 2's control.
 * Since Player 1 now controls two monsters, the effect of "Cyber Blader" instead doubles its ATK. "Snatch Steal"'s effect is no longer negated, so Player 1 gains control of the 'stolen' monster.
 * Since Player 1 now controls three monsters, the effect of "Cyber Blader" negates the effect of "Snatch Steal". The 'stolen' monster returns to Player 2's control.
 * The above sequence would repeat infinitely, with the player who controls the monster equipped with "Snatch Steal" never settling.

Since this loop was caused by a mandatory effect, it cannot be prevented by restricting the player's actions. Since the loop cannot be avoided, the card that started the loop, "Jam Breeding Machine" is destroyed by game mechanics before it can Special Summon the "Slime Token".

Loops with net change
Unlike loops with no net change, loops with net change cause a change to occur each time the cycle repeats. In these cases, the loop will continue until either the Duel ends or it ceases to have net change; if the latter, at this point the card ruled to have begun the loop will be automatically destroyed by game mechanics.

All-Out Attacks
All-Out Attacks OTKs utilize a loop with net change to force the opponent to continuously thin their Deck (either by drawing cards or sending cards from the Deck to the Graveyard), eventually resulting in a Deck out.

End by the Duel ending
Player 1 controls a face-down "All-Out Attacks" and a face-down "Morphing Jar #2" and "Hiro's Shadow Scout" that were Special Summoned. Player 1 has no monsters left in their Deck.

If Player 1 activates "All-Out Attacks", the following will occur:
 * Player 1's face-down monsters will be flipped face-up, causing both their effects to activate.
 * The effect of "Hiro's Shadow Scout" will make the opponent draw 3 cards.
 * The effect of "Morphing Jar #2" will return both flipped monsters back to the Deck, and Player 1 will then reveal cards from the top of his/her Deck until revealing two monsters. Since the only two monsters in Player 1's Deck are the two that were just returned, they will be the ones revealed.
 * The two revealed monsters will be Special Summoned face-down to Player 1's field.
 * Player 1's "All-Out Attacks" will then again flip the two monsters face-up, causing both their effects to activate.
 * The effect of "Hiro's Shadow Scout" will again make the opponent draw 3 cards.
 * "Morphing Jar #2" will continue to return the two monsters to the Deck and reset them face-down onto the field, where "All-Out Attacks" will flip them.
 * This infinite loop repeats until Player 2 has drawn close to their whole Deck, at which point the effect of "Hiro's Shadow Scout" will force Player 2 to draw when they do not have enough cards in their Deck. This will cause Player 2 to lose the Duel due to a Deck out.

End with no net change
Player 1 controls a face-down "All-Out Attacks" and a face-down "Morphing Jar #2" and "Needle Worm" that were Special Summoned. Player 1 has no monsters left in their Deck.

If Player 1 activates "All-Out Attacks", the following will occur:
 * Player 1's face-down monsters will be flipped, causing both their effects to activate.
 * The effect of "Needle Worm" will make the opponent mill 5 cards.
 * The effect of "Morphing Jar #2" will return both flipped monsters back to the Deck, and Player 1 will then reveal cards from the top of his/her Deck until revealing two monsters. Since the only two monsters in Player 1's Deck are the two that were just returned, they will be the ones revealed.
 * The two revealed monsters will be Special Summoned face-down to Player 1's field.
 * Player 1's "All-Out Attacks" will then again flip the two monsters face-up, causing both their effects to activate.
 * The effect of "Needle Worm" will again make the opponent mill 5 cards.
 * "Morphing Jar #2" will continue to return the two monsters to the Deck and reset them face-down onto the field, where "All-Out Attacks" will flip them.
 * This infinite loop repeats until Player 2 has milled their whole Deck, at which point the effect of "Needle Worm" ceases to cause the opponent to mill any cards. The loop now has no net change.

At this point, just as with infinite loops with no net change, the card that began the loop, "All-Out Attacks", would be destroyed by game mechanics. Once Player 1 ends their turn, Player 2 will be unable to draw a card during their Draw Phase and will lose the Duel.

In the anime
Infinite loops with net change have been seen a number of times in the anime:


 * In episode 67 of Yu-Gi-Oh!, Yami Yugi used the effects of Strings' "Slifer the Sky Dragon", "Revival Jam", and "Card of Safe Return" against him by taking control of "Revival Jam" by "Brain Control", forcing "Slifer" to attack and destroy "Revival Jam" over and over. This caused "Card of Safe Return" to make Strings continuously draw from his Deck until he Decked out and lost the Duel. This is arguably the most famous infinite loop to be seen in the anime.
 * In episode 182 of Yu-Gi-Oh!, Yami Yugi used the effects of "Legendary Knight Critias" (which had absorbed the effect of "Attack Guidance Barrier") and "Legendary Knight Hermos" to continuously redirect attacks between them while another effect of "Hermos" increased its ATK each time. By anime mechanics, this caused the ATK of "Hermos" to become ∞.

Controlled loops
These are not infinite loops, but rather a series of voluntary actions that can be repeated as many times as a player wants. There is no uncontrollable element to these loops and a player can choose to stop them at any time, so they are legal to use.

Some loops that require only a few cards (fewer than 4) to pull off have caused cards to become Forbidden or Limited on the current Forbidden/Limited lists, as otherwise their presence would be detrimental to gameplay.

Quillbolt Loop
This uses "Quillbolt Hedgehog's" ability to Special Summon itself by having a Tuner on the field along with a monster effect that Tributes a monster for damage such as "Cannon Soldier". "Quillbolt" can be Tributed again and again and continuously revived, as long as "Imperial Iron Wall" prevents "Quillbolt Hedgehog" from being banished by its own effect.

Flint Lock Loop
This uses "Flint" and two "Flint Locks" to pass the "Flint" back and forth. The Spell Card "Morale Boost" is used to gain 1000 Life Points every time an Equip Card is equipped. Because no Equip Card leaves the field, the effect to lose 1000 Life Point is never reached. This allows a player to gain an unlimited number of Life Points while the opponent may be powerless to stop it.

Gearfried Loop
If a player controls "Gearfried the Iron Knight", then it is possible to activate "Butterfly Dagger - Elma" an unlimited number of times per turn, by equipping it to "Gearfried", whose effect will destroy it, at which point "Elma's" effect will return it to the hand to be equipped again. This can easily be combined with "Royal Magical Library" to draw an unlimited number of cards in one turn (causing an easy Exodia FTK, etc.). Another option is to combine it with "Spell Absorption" to gain an arbitrarily high number of Life Points; adding "Fire Princess" gives an unlimited amount of damage to inflict in one turn on top of that. This loop is the reason why "Butterfly Dagger - Elma" is currently Forbidden on the Forbidden/Limited List.

Gate Material Loop
By combining "Fusion Gate" and "Chain Material", it is possible to Summon or activate nearly any card an unlimited number of times per turn, through the correct (and very risky) setup. These loops generally revolve around Fusion Summoning several Fusion Monsters by banishing Fusion Material Monsters from the Main Deck, followed by using said monsters to activate effects or Summon other monsters, then finally Fusion Summoning "Elemental HERO Electrum" to return the originally banished monsters back to the Deck to be banished again. For an example, see Elemental HERO Gustav Max OTK, an OTK that involves Summoning "Superdreadnought Rail Cannon Gustav Max" at least four times in one turn. For further and more detailed examples of possible setups, see this external link.

Primal Seed Loop
If a card is active that banishes Spell Cards that would go to the Graveyard (such as "Macro Cosmos" or "Banisher of the Light"), and there is a "Black Luster Soldier - Envoy of the Beginning" or "Chaos Emperor Dragon - Envoy of the End" on the field, then any player with two copies of "Primal Seed" and a Normal or Quick-Play Spell Card in their hand can activate that Spell Card an unlimited number of times per turn, by repeatedly using one copy of "Primal Seed" to bring back the other copy along with that Spell Card. This allows for an OTK with a card as weak as "Sparks", as well as numerous other possibilities such as arbitrarily high Life Points or ATK. This loop is why "Primal Seed" is currently Limited in the OCG; it was Limited in the TCG as well until 1 April 2014.

In the anime

 * In episode 64 of Yu-Gi-Oh! GX, Syrus used the mandatory effect of Hassleberry's "Ultimate Tyranno" against him, forcing it to attack and destroy "Decoyroid" over and over while the combined effects of "Ambulanceroid" and "Rescueroid" continuously Summoned it back to the field. This allowed Syrus's "Cyber Summon Blaster" to continuously damage Hassleberry until he lost the Duel.