That will really just ruin your day.
Guy had a double fire on his nail gun and thought that it had only fired one and kicked back and hit him in the face. Six days later he found that it wasn't the gun that had hit him, it was another nail.
Story to be found here:
I can't wait to start getting the calls from parents wanting their kids to wear some kind of armored mouth guards when using the pneumatic tools.
Also, you can't help but notice that the guy now has a $100,000 medical bill and no insurance. Keep that in mind next time you take a job 1099 - no workmen's comp.