How Could I Be So Stupid/Careless?

I was getting ready to hit a huge downhill jump on my Redline 20" pedaling my brains out, standing up. My chain was on the loose side and came off. My right kneecap and the beartrap pedal took some of the fall, the rest went through my nuts and the top tube.
Didn't wreck but couldn't bend that leg with the 2" knot sticking out of it for a while.
I always remembered to tighten the chain after that