This might seem as an already asked question, but in fact it is a suggestion. The related question is this one.
What I suggest is that when a question is edited, the asker (if he is not the editor) and the answerers get a notification. Also, if an answer is edited, the asker should get a notification too.
My argumentation is simple - the wiki nature of SO enables such edits, which enable the users to actually completely rewrite an answer or question, which has already been answered. Although almost everyone here has the discipline not to make drastic changes, I believe this will be useful because if the question becomes closer to the issue being asked for, this could render some answers either obsolete or less accurate. So, the entire thread will not be satisfying for a new reader with similar problems. On the other hand, if an answer improves, it may become the preferred answer to other ones and therefore be more-likely to get accepted.