It seems that when things move quickly enough, I can incorrectly lose the ability to change my vote on an answer.
Here's what happened (on this answer), all within ~15 minutes:
I commented that the answer didn't address the question, and downvoted.
OP edited the answer to address the question, but it was soon enough after the original posting that it doesn't show up as a distinct edit.
I deleted my comment.
I attempted to undo my downvote, and got a pop-up message:
You last voted on this answer
13 minutes ago
Your vote is now locked in
Unless the answer is edited
So, even though the answer was edited, substantially, after I downvoted, I can't reverse my downvote on the basis of that edit.