I accidentally voted to reopen a question that I just meant to make some improvement edits to. When I attempted to reclick the 'reopen' link I was told that I'd already voted to reopen and there was no option to rescind my vote.
Reopen votes can't be rescinded. Alas! Don't worry too much about it though. No harm's done, and this just happens sometimes so we take it in stride.
Four other people still have to agree to reopen it (or make exactly the same mistake), and other users can re-close the question if it really still needs work.