Yep, sounds like a the Pawns Stars misfired! :)
The Myth Busters did that once:
http://www.youtube.com/watch?v=ui-xBeqBNi4
http://www.cnet.com/news/mythbusters-make-apologetic-house-call-after-cannon-accident/
http://pleasantonweekly.com/news/2012/06/27/mythbusters-to-resume-testing-experiments-after-cannonball-mishap-