if a creature with vigilance attacks and its controller accidentally taps it because they forget, what happens. assuming its a competitive tournament. vigilance isnt a may, right? so game would just reverse?
The controller would have to untap it, I'd imagine. Since vigilance means they don't attack to tap, they would have essentially tapped it for no reason. Since there's no way to just tap a creature (without spells/abilities), it would have to be undone.
Of course, it's all up to how much the opponent wants to argue and ultimately what the judge says, but hopefully it wouldn't get to that point.
It's not a 'may' or optional effect. Assuming the mistake is caught fast enough (before irreparable gameplay has passed by), it would just untap, or at worst you would go back to the declare attackers phase and re-do the attack correctly.
If too much time has passed, and the game has pressed on too long before you realized it, you're pretty much jus out of luck. It'd be considered a missed trigger (essentially). Note that it's both players' responsibility to keep these things in line, and in a competitive REL event, you'd both get a warning.
thanks for the quick response guys. can always count on you.