I've never witnessed an accidental discharge, personally. Plenty of negligent discharges though.
The closest thing I've ever seen, or had happen to 'accidental', was cleaning a chinese double shotgun and forgot that I'd reloaded before doing the function checks and blasted a hole thru the ceiling. Hammers are smooth, not checkered, and upon lowering the right hammer, it slipped and said 'boom'. Surprisingly, nobody heard anything.
Property manager charged 75$ for the ceiling and told me not to tell anyone as it could hurt leasing sales... there's a story behind that one. Luckily, nobody was hurt.