No one else has admitted to this but I will.
I have loaded, chambered and fired a squib load through my .45, resulting in a round stuck hard in the barrel. After firing the non shot It took a second to sink in as my mind replayed for me in slo mo what had happened.
I had been doing some load development shooting through the chronograph and was on the 5th or 6th magazine being tested. Not wanting to "kill" the chronograph, my shots were deliberate and slow at this point as I was interested more in collecting speed than grouping data. Even with muffs on I knew that something improper had just happened with the last pull of the trigger. I did not hear the normal report or feel any muzzle flip. My first thought, "bad primer," was soon dismissed as I remembered hearing that distinctive primer pop and seeing a small curl of smoke from the gun. At the same time I realized that no spent case was ejected. I dropped the magazine, racked the slide back and removed the case. The primer had obviously fired. Then, with the mag out and the slide back I held the gun up to try and fill the barrel with light.... No light. Plugged.
I was shaken because I thought that my system was safe. That's another way of saying that I thought I was safe. Obviously, that wasn't the case. I reload on a single stage and my method to prevent errors includes many checks. Somehow, I screwed up. You can't check too much. Out of thousands and thousands of rounds, I missed a powder charge and didn't catch it, even on recheck.
Distracted, complacent, asleep at the wheel, hypnotized by habit? I don't know.
Keep checking. Don't believe that you or your methods can't be breached.