If you look at the revision list for this question (pointed out to me by The Cat), you can see I voted to close once, then there's a bunch of deleting and undeleting, and then I somehow voted to close again after it was reopened.
There have been a number of bugs lately with migration/close votes, and for this question Joel was able to vote twice in the same closing cycle.
I'm not sure if my being able to vote more than once on separate closing cycles is related to the other bugs happening recently, or if the one-close-vote limit somehow gets reset when questions undergo deletion/undeletion. Anyone have any ideas?
UPDATE:
I guess it doesn't have anything to do with deletion/undeletion, since I was able to vote to close this question again even though I had voted to close it (successfully) about 3 days (maybe just barely 4 days) ago.