A controlled loop is 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.
They contrast to infinite loops, which the players have no control over once they start.
This uses the effect "Quillbolt Hedgehog" 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.
A far more consistent variation of this loop uses "Plaguespreader Zombie". Summon both "Quillbolt" and "Plaguespreader" to the field, and Xyz Summon "Ghostrick Socuteboss", then activate its effect. Then, Summon "Ghostrick Angel of Mischief" by using "Socuteboss as the Xyz Material", then detach an Xyz Material (not "Socuteboss)" to add a generic "Ghostrick" Spell/Trap to your hand. Place this Spell/Trap back on top of your Deck to revive "Plaguespreader", then revive "Quillbolt" with its effect. This returns to the start of the loop.
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.
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.
Wicked Worm Beast Loop
If "Wicked Worm Beast" is Special Summoned in the End Phase of the turn it is automatically returned to the hand. Therefore, if a player has 7 or more cards in their hand in the End Phase of the turn and "Dark World Grimoire" active, they can discard "Wicked Worm Beast", Special Summon it with "Grimoire", and return it to the hand again. Although this has limited value, if the opponent activated "Maxx "C"", they could Deck out. Similar strategies can occur using "DNA Surgery" and "Cyber Summon Blaster" or "Soul-Absorbing Bone Tower", the latter of which works with "Zombie World". This loop is more easily achieved using "Silent Wobby" with a card that takes control of it.
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.
- In episode 98 of Yu-Gi-Oh! ZEXAL, Yuma Tsukumo and Astral fused into Dark ZEXAL against Vector, with the latter, who was blinded by rage, taking control. He equipped "Dark ZEXAL Weapon - Chimera Clad" to "Chaos Number 39: Utopia Ray V", granting it immunity by battle destruction and the ability to attack once again with doubled ATK whenever Dark ZEXAL took battle damage from attacks involving it. He then proceed to attack the higher-ATK "Number 104: Masquerade", taking the first damage and triggering the first ATK boost. However, Vector activated "Extra Hundred" in response to the second attack, preventing "Masquerade" from being destroyed in exchange for letting "Utopia Ray V" attack it a third time. However, by the second effect of "Extra Hundred", each time "Masquerade" would be targeted for an attack from then on, its ATK would become 100 points higher than the attacker's. Astral didn't realize the danger, and continued to attack with "Utopia Ray V" again and again, losing 100 LP each time, until he was down to 600. At that point, Yuma managed to cancel the fusion and promptly ended the turn, preventing the 500 damage of "Sargasso the Different Dimension Battlefield" from reducing their LP to 0. Had he not managed to, Astral would probably have continued the loop until he lost.
- In episode 99 of Yu-Gi-Oh! ARC-V, Declan Akaba activated the effect of "D/D/D Wisdom King Chaos Apocalypse" against Jean-Michel Roger, which lets him Special Summon it from his hand by destroying two face-up Spell/Trap Cards he controls, those cards being "D/D Savant Nikola" and "D/D Savant Thomas". Since "Nikola" was destroyed while it was in the Pendulum Zone, its effect let Declan return a "D/D/D" monster, "Chaos Apocalypse", on his field to his hand, and then place up to two face-up "D/D" Pendulum Monsters from his Extra Deck ("Nikola" and "Thomas") to his Pendulum Zone. Declan followed up with "Dark Contract with the Swinging Abyss", inflicted damage equal to the DEF of a "D/D" Pendulum Monster that was returned to his hand. As the DEF of "Chaos Apocalypse" is 2000, this was enough to wipe out the rest of Jean's LP. Attempting to bypass his loss, Jean re-entered the Duel and took the 2000 LP intrusion penalty. Declan revealed that he would simply use the effect of "Chaos Apocalypse" to Special Summon it again, triggering the effect of "Nikola", and in turn triggering the effect of "Abyss Pendulum", which would deplete Jean's LP again. Realizing that Declan would repeat this loop each time he attempted to re-enter the Duel, Jean forfeited and fled.
- In Episode 133 of ARC-V, Yuri activated the Ignition Effect of "Starving Venom Fusion Dragon" targeting Yusho Sakaki's "Performapal Sky Magician" to gain its effects. However, he then chained the effect of "Clear Wing Synchro Dragon", destroying "Starving Venom" and gaining its ATK. Yuri proceeded to activate "Flower of Destruction" and "Fruit of Destruction", with the latter reducing the ATK of "Clear Wing" by 3000 to Summon back "Starving Venom", and the former inflicting damage to Yusho, and reducing the ATK of "Sky Magician", equal to difference between current and original ATK of "Clear Wing", initially 200. As "Starving Venom" was just Summoned again, Yuri triggered the combo by activating its Ignition Effect again. The loop was repeated a total of 5 times, with the damage increasing by 200 points each time, before "Starving Venom" attacked "Sky Magician" and defeated Yusho.