The fact that he kept teaching the class is notable, but that is why I will never look at a gun with the bolt or cylinder closed. I had something like that happen on the trap range with my 12 gauge automatic. Thank God I always point the gun down range or away from others when I load, because I slipped a shell in the chamber, closed the bolt and the gun fired! The safety was even on, but the firing pin got stuck in place. I could've killed someone if I hadn't learned gun safety when I was little.
Needless to say that gun went straight to the gunsmith, and even though he didn't find any problems and it was probably a fluke, I still think about it every time I shut that bolt.