I have seen this aswell. The case was split and the bullet was lodged in the wooden bench. The primer stayed intact with the casing. No one even realized this happened until the shooter went to reload his next magazine.
I also keep my ammo boxed up till I need to reload just because of this. Sorry I don't have any pic's of the occurance I witnessed. I don't make it a habbit to carry my camera to the range.
Remember listening to Paul Harvey one day(pre Rest-of-the-Story days) when he told a tale of a hunter who had a round go off in his shirt pocket resulting in his demise. Hard a hard time believing that one.
Also, there was an article in Gun Digest where they were detonating centerfire rifle rounds with a soldering iron to determine what damage would occur. They would lay a bar of Fels-Naptha bar soap under the round, plug in the soldering iron (that was touching the primer) and back off. The exploding round would just barely dent the bars of soap.
Weird things can and do happen that seems out of the reality realm. I'd guess the case was dented hitting the roof structure too. The live load set off might be dependent of the box the round was housed in too? Imagine an ammo box milled out of aluminum? Wouldn't that also act as a chamber? Just when we think we've seen or know it all, we find out we don't?
I was at a big multi-day shoot that had a room deal with a really nice hotel. This was a plush hotel with an eight story atrium rising from the lobby.
When I left the shoot I absent mindedly put a .410 shell in the breast pocket of my shirt. As I entered the lobby and walked to the elevators, many stories above a wife and her husband were having a domestic dispute. She was in a rage. When she started throwing things he decided it was time to get out of Dodge.
As he dodged through the door she threw a Gideon Bible at him as her final round. He ducked, and it missed him, but it flew over balcony and fell the eight stories to the lobby and hit me in the chest. It knocked me unconscious. They took me to the emergency room, and I recovered fully. But the Doctor told me I was really lucky. If it wasn't for the bullet in my pocket, I would have probably been killed by that Bible. ;-)
Those pics don't prove a damn thing. There are numerous holes in that ceiling and the spent case looks like it was caught by the slide. I had an auto that use to catch the spent cases and dent them just like that. It likely caught the spent case and that pressure popped the primer out. I'm calling BS too. Hell all the evidence is there to prove the story as BS. I saw none to prove the story true.