Years ago during a handgun competition the front runner lost the match when one of the supplied rounds didn't go off. When they checked the round out they found the primer didn't have any priming compound at all in it. Basically is was just an empty metal disk. This was a factory shell by the way. I was about 10 or 12 when I read about it so that would have been about 1973.

I experienced nearly the same thing with a percussion cap on my muzzleloader once. I lined up on a decent mulie buck and all it did was go click when the hammer fell. I cocked it again and it went click again and the deer said bye-bye. When I took the cap off and looked at it, I noticed no priming compound in it either. I checked the rest of my caps and they were all fine. Murphy's Law dictated that particular cap was on the rifle while hunting and not of the range.

Primers are manufactured by the millions. Like anything coming off an assembly line there are going to be some defects and no quality control system will catch them all. It happens.