had one once with an AR-15...
somebody took a live 5.56round from my pocket while doing laundry (forgot it from a range trip), and put it in with what looked more like that round.. and that was my dummy 5.56 rounds box (swiss military dummies are made with brass) ...
Afterward, I loaded a mag with dummies for drills practice, and after 15min of drills, BANG... still I was following the 4 rules, and the round went into a big piece of wood I was aiming at.. my ears were strange for a day or two though..
After that event, I threw all my military brass dummies away, and bought nicered/orange plastic dummies..