If you hover over the upvote/downvote buttons on any website on which you don't have an account, the tooltip will explain the action but with the text
(click again to undo)
Which is very confusing. Undo what?
I've checked with Firebug, there seem to be only one Ajax request, which is when the button is clicked, but when you click it again, nothing happens. So nothing is undone.
Is this a mistake in the tooltip? A bug in the Ajax request script? Or is it by design?