I had a similarly scary incident a month or so ago. I was function checking some crappy steel mags (I know, don't yell at me) to see if any were worth keeping as range mags, or just tossing. I'd been having all sorts of problems, failure to feed, double feeds, triple feeds, bolt overs...
Anyway, I load a topped off mag, drop the bolt, pull the trigger. Click! I figure the bold closed without stripping a round, so I make sure the mag is seated, pull back the charging handle, and let the bolt drop again. This time the bolt stops about half way, not quite far enough for the FA to be of any use. Not the first time this had happened. I figure the mag could be overyly tight when full, maybe a bent feed lip. I pull back the CH and let the bolt drop 2 or 3 more times, same result, before I saw what was really happening.
The round wasn't stuck in the mag, there was a live round already in the chamber. The second round fed halfway out of the mag, untill the point of the bullet hit the first round about 1/8 - 1/16 inch below the primer. The bullet was jammed about a 1/4 inch back into the case from my repeatedly dropping the bolt on it.
I hate to think what would have happened had that round struck the primer of the LIVE round in the chamber, with the bolt open. Needless to say, the steel mags are trash and I'm only using USGI from now on.