[[File:Infinite loop.gif|thumb|200px|The famous infinite loop that allowed [[Yugi Muto|Yugi]] to defeat "[[Slifer the Sky Dragon (originalanime)|Slifer the Sky Dragon]]"]]
An '''infinite loop''' is a never-ending cycle of effects [[Activate|activating]] in [[Respond|response]] to each other, or a series of [[Continuous Effect]](s) that never settle.
==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.Previously, Upper Deck Entertainment rulings ruled that the player could not play a card if it would voluntarily cause an infinite loop.<ref name="UDE P-R">[http://web.archive.org/web/20090220140220/http://entertainment.upperdeck.com/yugioh/en/gameplay/faqs/cardfaqs/default.aspx?first=P&last=R UDE FAQ]: Individual Card Rulings [P-R]</ref>
These types of infinite loops are illegal in gameplayThe North and Latin American TCG Policy 2.1 amended this policy, meaning that allowing a player may not to voluntarily [[activate]] any a card if its activation or effect, or place a monster on the field in a way that would cause such a an infinite loop. If such a When an infinite loop is caused by something involuntary, such regardless of whether it was done voluntarily or involuntarily, it is first determined as to whether the loop would cause a victory condition; for example, if an infinite loop in which a Duelist draws 2 cards per iteration will eventually end in fulfilling a victory condition for one of the Duelists. If the infinite loop would ''not'' fulfill a player conducting their [[normal draw]] or victory condition, then each action in one iteration of the loop is performed a mandatory effectsingle time, and then the card ruled to be the source of the infinite loop is automatically destroyed sent to the Graveyard by game mechanics.If multiple cards are causing the loop, the most recently activated is sent to the Graveyard by game mechanics. Any effects that activate as a result of these card(s) being sent to the Graveyard proceed normally.<ref name="TCGPolicy2.1">[https://web.archive.org/web/20201118000152/https://img.yugioh-card.com/en/gameplay/penalty_guide/YGOTCG_Policy_v_2_1.pdf Konami Digital Entertainment, Inc. (KDE-US) Official KDE-US Yu-Gi-Oh! TRADING CARD GAME Tournament Policy - North & Latin America Territories]: Loops [O]</ref>
===Pole Position===
"[[Pole Position]]", "[[Luminous Spark]]", and "[[Gemini Elf]]" (1900 ATK) are face-up on the field.
If a player attempts to Summon Summons "[[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.
* The above sequence would repeat infinitely, with the ATK of "X-Head Cannon" never settling.
Therefore, it '''Old Ruling:''' The player is an illegal move for either not allowed to Summon "X-Head Cannon". '''New Ruling:''' The player is allowed to Summon "X-Head Cannon". As "[[Pole Position]]" is the card that is the primary cause of the loop, it will be sent to the Graveyard by [[game mechanics]] after the above sequence has been performed once.<ref name="UDE P-R"/>[http://web<ref name="TCGPolicy2.archive.org1" /web/20090220140220/http://entertainment.upperdeck.com/yugioh/en/gameplay/faqs/cardfaqs/default> "X-Head Cannon", the monster with the highest ATK after "Pole Position" has left the field, will then be destroyed by the effect of "Pole Position".aspx?first=P&last=R UDE FAQ]: Individual Card Rulings [P-R]</ref>{{fact}}
====Involuntary====
* The above sequence would repeat infinitely, with the ATK of "Muka Muka" never settling.
'''Old Ruling:''' Since this the infinite loop was caused by the unavoidable normal draw, "Pole Position" is sent to the Graveyard by game mechanics. Due to a mandatory actionspecial ruling, it cannot be prevented ''no monster is destroyed by restricting the playereffect of "Pole Position"''s actions. Since the loop cannot be avoided, '''New Ruling:''' As "[[Pole Position]]" is the card that is the primary cause of the loop, it will be sent to the Graveyard by [[destroygame mechanics]]ed after the above sequence has been performed once.<ref name="UDE P-R"/><ref name="TCGPolicy2.1" /> "Muka Muka", the monster with the highest ATK after "Pole Position" has left the field, will then be destroyed by the effect of "Pole Position".{{fact}} Under the old UDE rulings for "Pole Position", only ''playing a card'' could be disallowed. If "[[Banner of Courage]]" is the card causing the ATK fluctuations, the turn player may still voluntarily enter their [[Battle Phase]] and send "Pole Position" to the Graveyard by game mechanics]].<ref name="UDE P-R"/>
===Amplifier===
Player 1 [[control]]s a "[[Jinzo]]" which is [[owner|owned]] by Player 2, but being controlled by Player 1 due to "[[Snatch Steal]]". "Jinzo" is equipped with "[[Amplifier]]".
If Player 1 attempts to activate activates "[[Imperial Order]]", the following would occur:
* "Imperial Order" negates the effect of "Snatch Steal" ("Amplifier" cannot be negated).
* Since "Snatch Steal" is now negated, control of "Jinzo" returns to its owner Player 2.
* The above sequence would repeat infinitely, with the player who controls "Jinzo" never settling.
Therefore, it is an illegal move for '''Old Ruling:''' Player 1 is not allowed to activate "Imperial Order".<ref name="UDE A-C">[http://web.archive.org/web/20090217182013/http://entertainment.upperdeck.com/yugioh/en/gameplay/faqs/cardfaqs/default.aspx?first=A&last=C UDE FAQ]: Individual Card Rulings [A-C]</ref>
If '''New Ruling:''' After Player 1 had already controlled activates "Imperial Order" before activating "Snatch Steal"and causes the loop, then Player 1 activating "Snatch Steal" targeting "Jinzo" whichever card is identified as the cause will be sent to the Graveyard by [[game mechanics]] (equipped with "Amplifier"no official ruling has been issued as to which is the cause) would be an illegal move.
=====Involuntary=====
* The above sequence would repeat infinitely, with the player who controls "Jinzo" never settling.
'''Old and New Rulings:''' Since this loop was caused by a mandatory effectthe natural end of the turn, it cannot be prevented by restricting the player's actions. Since the loop cannot be avoided, whichever card is identified as the cause will be [[destroy]]ed sent to the Graveyard by [[game mechanics]] (no official ruling has been issued as to which is the cause).
====Negation====
* The above sequence would repeat infinitely, with which effect is active never settling.
Therefore, it '''Old Ruling:''' Player 2 is an illegal move for Player 1 not allowed to activate "Royal Decree".<ref name="UDE A-C"/>
If '''New Ruling:''' After Player 2 had already controlled activates "Royal Decree" before Player 1 activated "Skill Drain", then Player 1 activating "Skill Drain" would whichever card is identified as the cause of the loop will be an illegal movesent to the Graveyard by [[game mechanics]].
=====Involuntary=====
* The above sequence would repeat infinitely, with which effect is active never settling.
Since this loop was caused by a mandatory effect, it cannot be prevented by restricting the player's actions. ''Old and New Rulings:''' Since the loop cannot be avoided, whichever card is identified as the cause will be [[destroy]]ed sent to the Graveyard by [[game mechanics]] (no official ruling has been issued as to which is the cause).
===Cyber Blader===
Originally, the "3 monsters" effect of "[[Cyber Blader]]" could negate Continuous Effects, leading to the possibility of infinite loops. This was addressed in the ''OCG'' with the erratum issued in ''[[Dimension Box Limited Edition]]'', but persisted for several more years in the ''TCG'' until the same erratum was issued in ''[[Speed Duel GX: Duel Academy Box]]''.
====Voluntary====
Player 1 [[control]]s three monsters, one of which is [[owner|owned]] by Player 2, but being controlled by Player 1 due to "[[Snatch Steal]]".
If Originally, if Player 2 attempts to Summon Summoned "[[Cyber Blader]]", the following would occurhave occurred:* Since Player 1 controls three monsters, the effect of "Cyber Blader" negates the effect of "Snatch Steal". The 'stolen' equipped monster returns to Player 2's control.* Since Player 1 now controls two monsters, the effect of "Cyber Blader" instead doubles its ATK. The effect of "Snatch Steal" is no longer negated, so Player 1 gains control of the 'stolen' equipped monster.* Since Player 1 now controls three monsters, the effect of "Cyber Blader" negates the effect of "Snatch Steal". The 'stolen' equipped 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 '''Old Ruling:''' Player 2 was not allowed to Summon "Cyber Blader".
If Player 2 had already controlled "Cyber Blader" before Player 1 activated "Snatch Steal", then Player 1 activating "Snatch Steal" (targeting a monster other than "Cyber Blader") '''New Ruling:''' Whichever card was identified as the cause would be an illegal movehave been sent to the Graveyard by [[game mechanics]].
====Involuntary====
Player 1 [[control]]s two monsters, one of which is [[owner|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 Under the original effect, during Player 1's [[Standby Phase]], the following would occurhave occurred:* The mandatory effect of "Jam Breeding Machine's" mandatory effect Special 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' equipped monster returns to Player 2's control.* Since Player 1 now controls two monsters, the effect of "Cyber Blader" instead doubles its ATK. The effect of "Snatch Steal" effect is no longer negated, so Player 1 gains control of the 'stolen' equipped monster.* Since Player 1 now controls three monsters, the effect of "Cyber Blader" negates the effect of "Snatch Steal". The 'stolen' equipped 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. ''Old and New Rulings:''' Since the loop cannot could not be avoided, whichever card is was identified as the cause will be [[destroy]]ed would have been sent to the Graveyard by [[game mechanics]] (no official ruling has been was issued, but it will would have likely be been either "[[Snatch Steal]]" or "[[Cyber Blader]]"). ===In video games===Certain video games may implement their own rulings for infinite loops, or fail to recognize them leading to a crash{{fact}} or a softlock. For example, ''[[Yu-Gi-Oh! Ultimate Masters: World Championship Tournament 2006]]'' does the following:<ref>{{Cite web|url=https://www.youtube.com/watch?v=K7Ff82erD3E|title=Yu-Gi-Oh! Ultimate Masters 2006 Glitch Compilation - Loops & More! (World Championship Tournament)|date=October 2, 2019|website=YouTube|publisher=Scrub Busta}}</ref>* Both of the above "Cyber Blader" examples softlock the game. The monster equipped with "Snatch Steal" will display a never-ending animation of repeatedly changing sides, forcing the player to reset or turn off the game.* In at least the two voluntary "Amplifier" examples, the game will automatically destroy "Imperial Order" or "Royal Decree" after several loops in order to end the loop.* Interestingly, the infinite loop detection function appears to be built into (at least) the programming of "Royal Decree". For example, if a "Cyber Blader" loop occurs while "Royal Decree" is on the field, the game will destroy "Royal Decree" after several loops, even though it has no effect on the loop and the loop continues afterward as a softlock. In ''[[Yu-Gi-Oh! Master Duel]]'', if an infinite loop occurs (even if it has a net change), the game will generally stop it automatically somewhere around the 10th loop for those consisting of activations,<ref>{{Cite web|url=https://www.youtube.com/watch?v=tSm3-gYl_jc|title=Will This INFINITE LOOP BREAK MASTER DUEL? {{!}} Yu-Gi-Oh! Ruling Twitch Highlight|date=February 15, 2022|website=YouTube|publisher=DistantCoder}}</ref> or the 16th step if it is caused by never-ending Continuous Effects. In the latter case, one of the cards involved in the loop will also be destroyed.<ref>{{Cite web|url=https://www.youtube.com/watch?v=AOoG3XyrnJg|title=This INFINITE LOOP BLEW MY MIND! {{!}} Yu-Gi-Oh! Ruling Twitch Highlight|date=February 23, 2022|website=YouTube|publisher=DistantCoder}}</ref>
==Loops with net change==
* 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====
* 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 sent to the Graveyard 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.
===Red-Eyes Flare Metal Dragon===
This ruling can also be extrapolated to cover "Destiny HERO - Departed" with a nonzero ATK of 2000 or less while the player controls "[[Slifer the Sky Dragon]]".
===Light and Darkness Dragon=In the anime==The negation effect of "[[Light and Darkness Dragon]]" will only activate once while the effect of "[[Reverse Trap]]" is applying.<ref name="UDE L-O">[http://web.archive.org/web/20090220023927/http://entertainment.upperdeck.com/yugioh/en/gameplay/faqs/cardfaqs/default.aspx?first=L&last=O UDE FAQ]: Individual Card Rulings [L-O]</ref> In addition to preventing abuse, this also prevents anime shows some infinite loopswith net change used to end a duel.
If this ruling were not in place, infinite loops would occur with many mandatory Trigger Effects. For example* In [[Yu-Gi-Oh! - Episode 067|episode 67]] of ''Yu-Gi-Oh!'', [[Yami Yugi]] used the following would happen during effects of [[Strings]]' "[[Slifer the opponent's Sky Dragon (anime)|Slifer the Sky Dragon]]", "[[End PhaseRevival Jam (anime)|Revival Jam]] if that player controls ", and "[[Judgment DragonCard of Safe Return (anime)|Card of Safe Return]]":* The against him by attacking "Revival Jam", causing it to regenerate, then taking control of it by "[[mandatory effectBrain Control (anime)|Brain Control]] " so that while it regenerates, it does so on Yami's side of the field, forcing "Slifer" to attack and destroy "Revival Jam" over and over. This caused "Judgment DragonCard of Safe Return" activates to send cards make Strings continuously draw from the his Deck until he got a [[Deck to out]] and lost the GraveyardDuel.* The mandatory In [[Yu-Gi-Oh! - Episode 182|episode 182]] of ''Yu-Gi-Oh!'', Yami Yugi used the effects of "[[Legendary Knight Critias (anime)|Legendary Knight Critias]]" (which had absorbed the effect of "Light [[Attack Guidance Barrier]]") and Darkness Dragon" is Chained, negating the [[Legendary Knight Hermos (anime)|Legendary Knight Hermos]]" to continuously redirect attacks between them while another effect of "Judgment DragonHermos" and increasing increased its own ATK and DEF by 500each time.* The mandatory effect By anime mechanics, this caused the ATK of "Judgment DragonHermos" activates to send cards from the Deck to the Graveyardbecome [[∞]].* The mandatory effect In [[Yu-Gi-Oh! GX - Episode 064|episode 64]] of ''GX'', [[Syrus Truesdale]] uses "[[Decoyroid (anime)|Decoyroid]]" to force [[Tyranno Hassleberry]]'s "Light and Darkness Dragon[[Ultimate Tyranno (character)|Ultimate Tyranno]]" to attack it. After that card is Chaineddestroyed by battle, negating it's revived with the effect effects of "Judgment Dragon[[Ambulanceroid (anime)|Ambulanceroid]]" and increasing its own ATK and DEF "[[Rescueroid (anime)|Rescueroid]]". The net change in this loop is provided by 500"[[Cyber Summon Blaster (anime)|Cyber Summon Blaster]]" which inflicts 300 damage to Hassleberry every time "Decoyroid" is re-Summoned.* The above sequence would repeat infinitelyloop is maintained due to "Ultimate Tyranno's" effect that forces it to attack every monster once each, if able.
==In the animemanga==Infinite loops The manga shows an infinite loop with net change have been seen a few times in the anime.
* In [[Yu-Gi-Oh! Duelist - Episode 067Duel 117|episode 67Duel 117]] of ''[[Yu-Gi-Oh!Duelist]]'', [[Yami Dark Yugi (manga)|Dark Yugi]] used the effects of the [[StringsDoll (manga)|Doll]]' s "[[Slifer the Sky Dragon (manga)|Slifer the Sky Dragon]]", "[[Revival Jam (manga)|Revival Jam]]", and "[[Card of Safe Return (manga)|Card of Safe Return]]" against him by attacking "[[Revival Jam]]", causing it to regenerate, then taking control of it by "[[Brain Control (manga)|Brain Control]]" so that while it regenerates, it does so on Yami's side of the field, forcing "Slifer" to attack and destroy "Revival Jam" over and over. This caused "Card of Safe Return" to make Strings the Doll continuously draw from his Deck until he got a [[Deck Outout]] and lost the Duel. This is arguably the most famous infinite loop to be seen in the anime.* In [[Yu-Gi-Oh! - Episode 182|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 [[∞]].
==References==