As gleaned from a comment to a different meta question, I learned that close votes "expire" after a certain time period. It appears, though, that those who have voted to close a question are then prevented from voting again. Attempting to cast a close vote after the initial vote has expired results in a "you have already voted to close this question" message.
Is this expected/normal behavior? If so, it really makes appropriate community moderation difficult. See for example this question: Is file_exists() compatible with timthumb.php?
Obviously, it should be closed (TimThumb &
is_file_exists() - completely agnostic to WordPress), but it never did get closed. But I cast a close vote a year ago, and now can't again.