Ball is addressed, the backswing finishes and the down stroke starts and the ball settles in the grass or brush, player cannot stop the downward stroke to avoid hitting a moving ball. Had it happen to me more than once when playing from the second cut on a couple of good courses. Took a two stroke penalty on the hole. But that hole was a disaster anyway since my shot on the settling ball took a wild path from the club and the darn ball moved on the green when I was placing my coin to mark it.