The question that has made me think about this is What is an inexpensive and non-electric way to warm my sheets before getting in them?
If you look at the edit history, I initially posted an answer to the question before any edits were made. Then after I had left the question, OP added a constraint that directly invalidate my answer, which then led to my answer being downvoted.
My thoughts are that the answerer should not be punished, when the initial question is pretty clear (I agree that the fault lies with the answerer when attempting to answer a vague question, as stated in this meta post).
Should this sort of editing be allowed?
If so, should it be made more clear that the constraints were added as an afterthought?
If not, where do we draw the line between adding clarity, and changing constraints?
Would it be more appropriate to leave a comment on the answer adding the constraint, and waiting for a more appropriate answer to accept