Based on this answer to a previous question, I started monitoring my Reputation page. A couple of times I've seen an upvote appear with no reputation, only to have it disappear again when I refresh the page. Checking the answer reveals that it did indeed get an upvote, and the vote wasn't rescinded.
Update: there might be more conditions necessary to reproduce this bug. When I first noticed the bug it was on an upvote to the answer that was already at the top of the list with a reputation attached. I just received an upvote to another question and it is not disappearing, as expected.