Infinite loop
An Infinite Loop is a never-ending cycle of cards trying to resolve.
Because of this, a ruling was introduced that states you may not activate a card knowing that it will cause an Infinite Loop.
See Pole Position's rulings for examples of Infinite Loops.
If Ultimate Tyranno attacks a face down Spear Cretin and your opponent has another Spear Cretin in the graveyard the flip effect will special summon Spear Cretin into face down position and Ultimate Tyranno will have to attack it. The result is an Infinite Loop.
If you have Bad Reaction to Simochi and Prime Material Dragon on the field, any effect that would increase the opponent of the controller of Simochi's Life Points could not be activated.
Another one is if you have Manticore of Darkness on the field and activate Generation Shift and send Manticore to the graveyard and replace it with another one from the deck, then in the end phase you can cause an infinite loop by tributing Manticore to summon the other Manticore from the Graveyard and then keep activating the Manticores' effects. This is especially popular in an Exodia deck with Card of safe return on the field, which would guarantee victory provided all the pieces are in your hand/deck.
