Specifically on this question there is an answer that at first appeared to be correct, so I upvoted it. But after a while I discovered an error. I want remove my upvote and give a downvote instead.
It seems that by the nature of this particular (puzzling) stack exchange, realizing a part of the puzzle at a later time would be common, no?
Edit: After more exploration, it turns out I was mistaken and that answer is good after all, but hypothetically, couldn't there exist cases where a late downvote should be acceptable?