At the last Modern night at my LGS I was playing against a player who forgot to remove a counter from Ancestral Vision during his upkeep. It had 3 counters left on it and he remembered just after I drew after his turn and removed the counter. I decided to let it slide since there was no real "butterfly effect" between when he forgot and when he remembered.
I commonly hear players say if a card doesn't say "may" and they forget then it doesn't matter and they still get to take the action when they remember. Does this "ruling" hold up in an MTG/SCG sanctioned tournament? If it does, how far beyond from when they forget is allowable?
Relying only on the comprehensive rules, which cover both unsanctioned and sanctioned games, I would consider a player drawing a card without taking a mandatory action during that player's upkeep, such as putting a triggered ability with a mandatory action on the stack, as an illegal shortcut (C.R. 718.2a allows only shortcuts with sequences of game choices that can be legally taken), so that declaring such a shortcut is an illegal action that must be reversed (C.R. 719.1). As a result, in unsanctioned games, if a player realizes he or she has taken an illegal shortcut as I just described, the game goes back to the upkeep, when the triggered ability is put on the stack.
But admittedly, this illegal shortcut is problematic, especially if the active player has looked at the illegally drawn card, since actions that involve drawing a card can't be reversed (C.R. 719.1). If the illegal action is discovered only after the player's next turn begins, this can also be problematic, especially if the game state has changed in a non-trivial way (which I believe is what you mean by a "butterfly effect").
EDIT (July 27, 2016): Some rules were renumbered with Eldritch Moon.
I commonly hear players say if a card doesn't say "may" and they forget then it doesn't matter and they still get to take the action when they remember. Does this "ruling" hold up in an MTG/SCG sanctioned tournament? If it does, how far beyond from when they forget is allowable?
But admittedly, this illegal shortcut is problematic, especially if the active player has looked at the illegally drawn card, since actions that involve drawing a card can't be reversed (C.R. 719.1). If the illegal action is discovered only after the player's next turn begins, this can also be problematic, especially if the game state has changed in a non-trivial way (which I believe is what you mean by a "butterfly effect").
EDIT (July 27, 2016): Some rules were renumbered with Eldritch Moon.