I'm thinking there's a program in the background becoming re-activated (and on the 'foreground'). I had this just once, a few weeks ago. I think i heard a little beep or something of a program (could be norton, or snooper, not sure) and the key got stuck. Had to minimize and shit.
I'm not sure what I did that made it normal again. I probably activated a different program (winamp or so) and then went back to worms.