I recently posted an answer to a users question, which at that time was valid. The original poster later edited the question with a specific condition, which deprecated my (and several other users) answer(s). What is the rule of thumb in a situation like this? I guess the best thing to do is to update my answer to satisfy the newly added conditions. But what if I am not able to do this? Do I delete my answer?
I agree with Ayman Hourieh:
If edits are made after several answers have already been added, I'd argue the edits should be rolled back and a new question should be started instead.
provided that the original version contained a clear, reasonable question (which just happened to be not the question that OP intended).
On the other hand, if the original question was unclear, and the answers were based on guesses at what the question asked, then the answerers have only themselves to blame: they should have commented and/or voted to close as unclear instead of entering a speed-typing competition. The purpose of putting questions on hold as unclear is precisely to avoid creating a mess like this in the first place.