From wiki: "A slamfire is a premature, unintended discharge of a firearm that occurs as a round is being loaded into the chamber."
I learned what the term meant one fine day at an outdoor rifle range - this kid had a very tac'ted out, but dubious/beatup/rusty SKS rifle that he had purchased at a gun show. At the moment of truth, he was afraid to shoot it for the first time - I offered to make the first shot for him, figuring "What the hell could happen ? Either it goes 'Bang' or it does not".
Well, the rifle had 30 rounds loaded (it was a Chinese D model with an AK mag) , and it went full uncontrolled auto until all of 'em were fired - I just held the rifle as hard as I could, down range, and sweated it out until it was all over. Thankfully nobody got hurt, but I sure got an earful from the range boss

Lessons learned :
- Never shoot a stranger's weapon
- Always CLOSELY inspect any firearm you shoot - both inside and out.
- Always make sure ONLY one round is loaded when test firing for the first time.
- Always check that a free floating firing pin "rattles" before loading.
- Never volunteer

Slamfire