Call of Duty COD Invisible Glitch?

I was round my friends house, when we discovered he could not be killed on COD 7. I recorded a video of it, but would like to know why you think this happened. My friend could not be killed and was invisible. He could not run, however, he could have three guns. Please could you give your opinions on this matter.

The video is on youtube : http://www.google.co.uk/url?sa=t&so...tBKxysqwsTlZf9cIQ&sig2=MEl-22jb_caOM9GyuIFUUw

Wierd, nice share. Would have been better had this occurred in a multiplayer match on Nuketown. :'P