It was a Thompson Center muzzleloader, and the first thing I noticed next to the rifle was a small box that he'd poured most of a pound of black powder into to load from...
No, that's not confidence inspiring.
First, 'Don't do this' and make sure it's back in and the bottle closed. Then try another cap, no good. He had no nipple wrench, so finally had to go out to the truck, get a crescent wrench, and get it out. Well, whoever had last used it, when they finished, and cleaned and oiled it well before putting it away, and the flash passage was full of oil. Do the best can cleaning the passage, and since could tell there was still stuff in there, use an old trick of getting as much powder as could into the passage, then putting the nipple back in, and then capping and trying. That worked. And he had no further trouble the rest of the day.
Yes, he also got a bit of "How to clean after firing black powder" lesson.
I think I was the only guy working that day who knew anything about black powder arms. Part of me thought "And I'm your expert, you poor bastard." But he was happy and had a good day from then on.