Recently Michael Campbell was involved in a Rules incident on the teeing ground.
-> You can see the episode at this link (Facebook).
As you can see he made a practice swing approximately 5 inches behind the ball and accidentally struck it!
Two things could be considered:
- Was it a stroke? Answer: No, since he had no intention to strike and move the ball.
- Did he incur a penalty? Answer: No. The ball was not (yet) in play.
So he could place the ball on the tee again, and play his first stroke.
If the same thing had happened on the fairway, though, he would have incurred a one stroke penalty (since his ball then would have been in play) and should have replaced the ball.