I posted a question on Server Fault, and looked at my profile to realise that I only had a 75% accept rate. So I went through looking for which question/s had missed out.
It turned out it was this one: http://serverfault.com/questions/123673/set-up-basic-windows-authentication-to-connect-to-sql-server-2008-from-a-small-t
Since the only answer was unhelpful, I went to downvote it. There was no indication that I'd placed a vote previously (no up or down arrow overlaying the vote counts) - yet when I clicked the "down" arrow, I received a message saying "You last voted on this in April, you can't change your vote unless it is edited." But I haven't voted, as far as I can see!