Worst I've seen so far...
Last spring, I was shooting pistols at an outdoor range. There was a guy two stations over with a mix of stuff in a range bag. He'd finished firing a Ruger auto, and then brought out a little .25acp. I was just finishing loading up mags when I saw him put in a fresh magazine, chamber a round and try to shoot.
He pulls the trigger. Nothing happens.
He pulls the slide back, but the round won't eject. Later I was to discover that the round was stuck in a dirty chamber and the extractor wasn't functioning. In any event, he then proceeds to flip the gun over so that it is pointed directly at his own navel... the muzzle mere inches from his belly. He starts pulling the slide back and letting it fly, over and over, very fast.
I quickly backpedaled a few feet to make sure I was totally clear, and then yelled for him to stop. After a few anxious seconds, he heard/saw me and stopped what he was doing. I went over and cleared the weapon for him, explaining that if there had been a problem with the firing pin or any number of other mechanical failures, he'd have just shot himself... not to mention the fact that he'd just violated the range rules.
He didn't say much in response, but quietly packed his gear and left. I HOPE that the reality of it sunk in and he understood what could have gone wrong. I also made a point of talking with the guys working the range to keep an eye out for him in case he came back... mostly for his own good.