Mine does this once in a while in a game I play . . . I think it goes away on it's own, and probably has something to do with holding down shift/ctrl on the left hand side of the keyboard for some reason. If you hold them down for 6 or 7 seconds does it go away?
Also, just as an FYI, when it does it you can use Shift-2 (or was it shift-6) to make a ?. I still haven't figured out how to stop it from happening, but it is really annoying.