I've had primers fall out of my loaded rounds before. Years ago I was at a match, I used to put my reloaded rounds in a zip lock baggie. I picked up the bag and noticed what looked like dirt in the bottom. It was unburned powder, I also noticed a couple of loose primers on the bottom too. They had just fallen out of the pocket. I remembered during loading that a few went in easy, too easy... Hasn't happened since.