There's a current case of an infrequent problem: Why is Donjon offline?
Dakeyras's answer was a response to a question that was once there. It was made in good faith, and is a helpful answer.
The part of the question it was responding to no longer exists, and if the question were asked in this state to begin with, Dakeyras's answer would be not an answer to any question being asked, though it's helpful.
This pattern occurs sometimes: a question is asked, answers are supplied, the question is edited, and those answers are no longer responding to the current question and may sometimes now be completely irrelevant.
What do we do in these cases? Let an irrelevant answer sit there? Ask the author of the answer to delete it? Flag it for moderator attention?