In this game:
http://www.chessgames.com/perl/chessgame?gid=1268980
At move 36, Magnus Carlen has his queen hanging. It would have been most approriate for his opponent to let him take back that terrible move!
He somehow won after that, but that must be completely unrelated.
If player X hangs his queen, when, if ever, is player Y obliged to inform player X of his miscalculation?