Sorry to re-hash an old thread, but I thought I'd share a (somewhat) similar story - with an important lesson re-learned:
My buddy and I were at the range, shooting pistols. I was shooting an XD .45, and he had his well-used Auto Ordnance 1911A1 .45, which we've both shot on numerous occasions before. We're both shooting our targets, stopping, reloading magazines, and continuing. We're both reloading, and I'm only slightly aware that he slaps a magazine into his pistol, and drops the slide.
<BANG!>
I look over at him, and he's got this incredulous look on his face, staring at the pistol. Yep, it slam-fired. We look at each other for a good long second, until he says, "Well... THAT ain't supposed to happen!". Me: "Uh - nope". (I know - I've got such a way with words). While we were both relieved that he had the pistol pointed downrange when chambering a round, we were both extremely aware of what COULD have happened had that not been the case.
So, yes - this was one of those rare times when it can really be called an ACCIDENTAL discharge. Lack of negligence saved him from property damage, personal injury, or involuntary manslaughter.
Afterwards, we both handled the UNLOADED pistol, and dropped the slide several times (still pointed downrange, of course), and a couple of times, the hammer would fall.
After disassembling and examining the pistol, the notch on the hammer was pretty worn. Whereas on a new hammer, the notch forms an almost perfect 90° angle for the sear to enage, the notch on this hammer was worn to maybe 70° or so. The sear, instead of being a nice crisp, squared-off edge, was slightly rounded off. In short, both hammer and sear were "well worn".
Now, I can't answer why the sear didn't catch the hammer at half-cock... it certainly should have (seeing as that's the whole point of the half-cock notch to begin with) - but it just goes to show: NEVER rely on any mechanical safety device. ALWAYS assume the gun will fire when chambering a round. If I only understood that concept at an academic level before, it sure was driven home that day at the range.