How a primer got in. Maybe he stores his primers on a shelf above the loading bench and they spilled out one time when grabbing them to use. Maybe the primers fell, and he didn't notice one fell into the hopper. 
I'm not sure why a primer might have fallen into an open hopper, or open power can, but I'm also not sure why so many feel it's an impossibility. I've heard of several things happening to people reloading, and often wondered how it happened, but not because it never happened to me. Mainly because it just seemed unlikely I'd ever have the problem.
Same thing here. I doubt I'll ever have this issue, as I use a hand priming tool, and usually sit in the family room and watch TV as I seat primers. But it brings up a good thought to me. I'm going to use more care to look into my powder hopper before I just pour powder in for the next reloading session. Might not ever see anything, as I leave the lid on, even when it's empty. But I'm still going to make a conscious decision to check.

marlinguy wrote on Dec 20^th, 2014 at 12:42pm:


Back in around 1960 one of our club members was loading a Brown Bess (IIRC). Hee had just fired it and charged the measure on his powder flask with (black) powder.

He released the charge into the muzzle, but there must have been something smouldering at the breech-end, and there was a WHOOMP! and the flask split open flat, and all we could see was a big cloud of smoke with a pair of feet sticking out of the bottom. As the smoke cleared his blackened figure appeared, with a pair of rather surprised white eyes peering out of the grimy apparition. His eyebrows and eyelashes took some time to regrow, but other than that he was unhurt.

A closer shave was had by a friend and his son. The friend had been reloading ·357 Magnum and had loaded several ·38 Spl cases by accident. He removed these from the batch and put them in his pocket - and forgot them.

Months later on the range he loaded his ·38 Spl with the rounds he found in his pocket and gave the revolver to his son to use. The first shot split the cylinder into shrapnel, one piece going through a sheet of heavy-gauge (air-raid shelter) corrugated iron.

I have the (half) cylinder somewhere as an awful warning.

--
Aonghas