Someone please explain this ???

Has anyone seen this youtube video where a DEA agent accidently shoots himself in the foot. Can someone please explain to me how this accident happened? Obviously he pulled the trigger, that I know. After cocking back and a dry fire (which he does) there's not suppose to be 1 in the chamber, right? Where was this 1 round hidden .. where did it come from? Anyone?

Here's the link:
http://www.youtube.com/watch?v=eP6UvNgbqIA&feature=related

Don't worry, its not graphic nor is there any blood shown. You dont even see the gun go off, you just hear it.