I had a Jennings J-22 slam fire on me once. Very unnerving. I had been to the range that day, fired a bunch of weapons, the little Jennings amongst them. They were all placed empty into a haliburton case then brought home. I then cleaned most of them but hadn't gotten to the Jennings yet when I realized a nearby grocery would soon be closing and I needed to buy some milk before it did. My 38 was still dis-assembled for cleaning so I just grabbed a loaded magazine and loaded the Jennings. [In those long-ago days I used a wallet holster in my back pocket.] As I always had I racked the slide to chamber a round (with my finger nowhere near the trigger) when the gun suddenly fired. Some fuss ensued. Later exam of the fired .22 casing (still in the gun) revealed a strange dent in the case rim and a bulge of the case. Exam of the gun disclosed that sometime during the day's firing a shaving of brass had somehow wedged the extractor in the down (inward) position.
As most here know the extractor is supposed to just slide over the rim of the cartridge case, then hook it upon a firing and pull the case out of the chamber. In this instance instead there was no give to the extractor and it became a firing pin when it slammed against the rimfire cartridge rim. A total UD experience. No one hurt, but it got my attention. That was the end of my routine use of the Jennings and rimfire as a back up piece and a 'new' WW2 era PPK using centerfire cartridges replaced it within a week.
With hindsight, either one of two simple rules not followed could have prevented that slamfire. 1) Clean any gun that has been fired before loading it. 2) Inspect your weapon before loading it, even if it was just fine earlier that day.