For question with more than one answer with the same number of votes, it can happen that the vote for an answer ends up at the wrong answer. I think this happens if a partial update or reload changes the order of the answers for some reason.
I was hit by this more than once, and I start to find it really annoying. Because I can only change my vote if the answer has been edited, there is no obvious workaround for this bug. (I see this behavior with Safari Version 5.1.7 (6534.57.2), but I'm not sure whether this is the only browser affected.)