Today, I replied to a post. It was the first reply. The post wasn't very clear and I made a guess as to what the OP wanted. It appeared that he didn't understand array indexes. However, as the question "evolved" or was edited to become clearer, my answer slowly became irrelevant and to someone just browsing it may appear irrelevant. I'm guessing that's why someone downvoted it. Now, I don't care too much about voting or scoring, but I do care about answers becoming obsolete.
If questions can be modified and there is no active "conversation" of that modification it can be misleading.
How does Stack Overflow prevent "evolving post syndrome." Maybe allow the answers to be tagged as no longer relevant?