Well I know my dad when I was a kid was going to trade a guy for a muzzle loader, he never did much with them despite owning plenty of cartridge guns. Well the guy he was buying or trading from loaded it up for him to test fire it. When fired it peeled like a banana, octagon barrel split back from muzzle to about 6in. from the action in three fairly even steel splits. Amazingly my dad was not hurt, must have been holding things just right etc, but that was the last time he shot a muzzle loader and I don't blame him. We're not certain what happened, obviously something on the loading process, either double charge, maybe double bullet, something. But it was a good lesson that fortunately didn't cause an injury.
Now I'm not against muzzle loaders, never much spent time with them, can't say I've fired one honestly. I'd be happy to own several at some point just haven't gotten around to it yet. But it's a good reminder that you need to be careful, much the same way as loading your own centerfire ammo, it can be dangerous and it's best to do it yourself so you know what the heck is done;)
Luke