I came across a man snapping caps on a new CVA "Kentucky" rifle during deer season once. I stopped to see if he needed help, and sure enough he did. It was a brand new rifle - a gift from his son - and he had never fired it before. Or cleaned it. The breech was clogged with grease as it turned out, and a truly lucky thing that was! The gentleman was in his 60's, and he had gone to the store where his son bought that rifle for some advice on loading it. The clerk had sold his son a powder flask with a 60-grain spout on it and a pound of powder, a sack of .440 round balls and some pre- cut patches, and a tin of percussion caps. When the new owner asked about how much powder to use, the clerk told him "Put 4 or 5 of these spout fulls in. You can't over-charge a muzzleloader. If you put in too much the extra just blows out of the muzzle."
I had a ball puller that fitted his ramrod in my hunting pouch, so we pulled his charge. When we got the patched ball out I tipped the rifle and dumped the powder out onto a big leaf --- and dumped --- and dumped. My best guess is, he had loaded close to 400 grains of powder into that rifle. I looked at it, then poured a little into my palm from his flask. Yep! It was all FFFFg!
We got his rifle cleaned --- I had a jag that fit his ramrod too --- and I whittled a "field-expedient" powder measure from a piece of cane I cut nearby, and trimmed it to take 50 grains. My belt pistol uses 25 grains, so two of those. Loaded the cleaned rifle and he fired it at a mark right there. When I was sure he had the loading sequence down, I went on my way, having given him enough 2Fg from my horn for a few charges and explained why the clerk's advice was wrong AND DANGEROUS!
He was horrified.
A couple days later I told the NH Field Rep for the NMLRA about that incident and he took it to the NH Fish & Game folks. The result was the beginning of the Muzzleloading Hunter Safety courses in New Hampshire, which spread to other states in New England and was finally adopted by the NMLRA as their own ML Hunter Safety Program nationwide, all because some clerk repeated an old myth about black powder and the customer got lucky and survived the experience.
My father used to say, "Y'know --- Every once in a while, even a blind hog stumbles across an acorn." I expect this was the kind of acorn he was talking about.