Allen (who speaks from experience!) is correct. I think Davis Love III once hit his ball with a practice putt, then did not replace the ball to its original spot, but instead played it from the spot it rolled to. Whoops... 2-shot penalty for playing from a wrong spot.

Of course, if the ball is not yet in play, no penalty. So don't worry if the ball's on the tee and you knock it off while doing your waggle.