I've seen this happen once because a guy thought he knew more then anyone else and cross threaded his nipple into the drum. He would not address the problem of retapping the drum, (even the possibilties of installing a new drum) - but rather claiming, "it's fine,,, the nipple's in tight."
A few shots later he blew his nipple out - never to be found again. He also took a pretty good spray of gasses. He also quit shooting muzzle loaders. (Thank goodness)
To be honest, when he stopped shooting muzzle loaders all of us felt a bit more safe. He was just one of those folks that knew it all, and the rest of us were dummies, no matter what good advise we offered him.
Why did he cross thread his nipple? Most likely because he was constantly having misfires and pulling his nipple quite often to put a little powder in the drum to get the rifle to go off. He seldom cleaned his rifle, and he never (I kid you not) used a lube or spit patch. 99% of the time he pounded the dry patched ball down onto his charge...
He simply refussed to accept his muzzle loader as a muzzle loader with all the requirements and responsibilities that come with shooting these great rifles. I think the only thing that kept him coming back each month was that he would accidently hit the target every once in awhile.
I hate to say it, but him blowing out his nipple was the best thing that ever happened for our group of ML shooters. As I said, he never came back to another one of our shoots, and he just would not take any advise whatsoever on how to propperly clean, load, and maintain his rifle. Good riddens, period!
Some people are just lost to stupidity. He was one of them.