There’s a shocking amount of players that aren’t 100 percent sure what to do when this happens. Simple: You get to re-tee without penalty. (Exception: You’ve already whiffed on the first shot. If the ball then falls off, you have to play it as it lies.)
On the bright side, since things are a tad more serious in tournament play, you won’t have the jacka$$ who chirps “One!” when your ball falls off the tee. That guy sucks.
