My guess about the "mystery bullet" : mag was pulled but it stripped a round that was partially jamming slide and fell out enabling the slide to impact the cartridge that was not fully chambered. This video examples at least 10 unsafe gun handling rules.
The idiot gun handler should be charged and prohibited from access to guns, period.
I took the first Glock comment in jest. Second a person has to break several rules to hurt someone like that. I don't have to single them out for this crowd. Finally the guy behind the counter and/or the owner does have some guilt in as much as he should know or have been taught "situational awareness". After all, everyone knows that Idiots go in stores and such with loaded weapons. Rules and training of SA should be a constant reminder and every employees responsibility.
This should have been noticed when the guy pulled his loaded weapon and stopped right there.
The ranges I go to do not allow the employees or a customer to pull their own weapon.