I had a similar issue occur with a factory reconditioned Czech CZ 52 pistol.
When I got it, I tore it down and cleaned it, thoroughly, since it had been in storage in Czechoslovakia since being reconditioned in the 80's.
I took it to the range, and put a magazine in. When I pulled back and released the slide, it fired, twice. :yikes:
Thank God my dad hammered gun safety into me as a kid. It was pointed down range and the range was clear. It could have been catastrophic if that had happened somewhere else, or if I hadn't kept the muzzle pointed down range. The Wolf Gold FMJ ammo I had in it can hit 1,720 feet per second, with 570 foot-pounds force - I'd have hated for those two rounds to have hit someone. Instead, it just left me and my son, standing there with our jaws hanging down, wondering WTH just happened. The CZ 52 went back into the range bag. My CZ 52 was purchased not as a carry gun, but just because I like CZs and have a few different CZ pistols in my small collection.
It turned out that somehow, the firing pin was slightly longer than it was supposed to be (poor quality control when it was reconditioned, perhaps?) and was just long enough to impact the primer, and discharge the first two rounds when the slide seated. The third round had a slight dimple, but didn't discharge. I removed all the cartridges from the mag, and put the dimpled one back in, and fired it, because a primer that has been hit can be dangerous. The firing pin issue has been corrected.
This should be a reminded to ALWAYS pay attention to where that muzzle is pointed when loading and unloading a firearm.
Odd - since I got my Spyder, I haven't gotten any more guns - it seems that farkles suck up all the money that used to be my "gun fund!" :dontknow: