I had one of my SKS do that! Scared the SHIT right out of me!! Took it apart cleaned the crap out of it and no more slam fire.
Yeah.....years ago I had an SKS that I bought used from Green Mountain Guns. It slam fired on me as well. I was at the range with it pointed in a safe direction of course and blamo....never really trusted that rifle much after that I must say.
My determination to not strike the first blow, but also to not stand mute and allow myself, and all that I believe in and stand for, to be trampled by men who would deprive me of my God-given, or natural right to suit my own end.