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: Set up basic Windows Authentication to connect to SQL Server 2008 from a small, trusted network
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!