Here is a URL where you can see a picture of the Breckenridge carpenter’s x-ray and hear an audio of him explaining how he nailed himself in the face without knowing it.
Basically, he stumbled, hit himself in the face with what he thought was the back of his nail gun and when he noticed blood, thought he had hit hard enough to split his lip.
http://www.msnbc.msn.com/id/3842331/
BruceT