I believe it's the interraction of the forum script and cookies stored on the pc, to prevent (in the case of forums) naughty people from multiple/spam posting or pm'ing. Probably triggered by logging in twice e.g. by using the back button instead of links.
Most commonly you see it on voting / reviews of products, holidays etc. If I sell widgets online and 50 customers think they're crap and award 1 star, by altering the script I can suddenly turn that into a 5 star rating just by making one vote.
Either refresh the page or log out and log in again (but using the proper logout / sign in links rather than the back and forward buttons) and all should be fine again.
J
