I downvoted something, assuming it was some piddling info-post I'd made, when it was actually the post beneath mine. It wasn't particularly offense and not obviously worthless, but I can't upvote to balance things back.
Updated
Posted under General
In the past this wasn't possible due to the way votes were logged (or the lack thereof), but all you had to do was wait for someone else to vote, and you could eventually recast your vote. It was primarily the abuse of that feature actually that lead us to where we are now.
Now that a larger number of usernames are maintained for each posts votes, this may be technically feasible now.