I was unlucky enough to see a guys flintlock go off just as he was seating the ball. The 3/8 fiberglass ramrod went thru his hand, then thru his upper arm and then thru the roof. The .50 rb went thru his hand and ended up on the backside of his wrist. He did survive and after many months of PT was alright. WHat happened? Well after lots of thought it seems the frizzen must have been closed and the hammer back. It was an older lock (late 60's, early 70's vintage) and wasn't the highest quality. Half cock on that thing was as almost as far as my SIler full cock. Six months earlier I saw a lady have a T/C pistol go off. Had cleaned the gun and forgot to snap the last cap off after snapping caps on the cleaned gun. She didn't get hurt bad. Slight cut/burn in palm of hand from ball starter.
Those are tragic things for sure, and lucky there was no more serious damage done...and I appreciate seeing the detail because deep in my mind, while I still keep my head away from muzzles, I have never really bought the fact that people have had spontaneous discharges without a preventable cause...ie: the lock condition and position in your example.
IMO, there are probably cases where folks are certain that there was no cause like the one in your example and the rifle "just went off", but I doubt that's reality...there always has to be an ignition source and I believe human error is way high on the list of causes.
BP needs to be respected of course but it's not nitroglycerine...they ship cases of Goex to me cross country in a cardboard box...thanks for posting