Been meaning to pop in here to mention that I've seen this occasionally, too. A quit/restart has always fixed it for me, and there doesn't seem to be much pattern to it (I could be playing the game, quit, restart to find the "P" problem, and then quit/restart once more to clear it up, etc.)
Quite odd. I wonder if it has anything to do with the renderer? Since I'm on Linux I'm obviously using OpenGL, but I wonder what the Windows folks who've seen this are using.
Not a big deal, of course, given the simplicity of the workaround.