I've never had one, but I've seen two. Both side by side shotguns, but I don't think that had anything to do with it.
One the kid had poured the powder and shot out of a bunch of shotgun shells, then popped the primers. The underwads apparently got stuck in the barrel. When he loaded a live shell the barrel ruptured.
The other one, a fellow showing off poured in a BUNCH of black powder into a percussion shotgun. He was telling us, "You can't overcharge one of these things. With no shot charge, just the powder, he capped the gun and fired it. The barrel ruptured right at the forearm. I thought he'd blown his hand off.