atx power connector cable burned and melted - what did I do wrong?

perspective11

Distinguished
Aug 20, 2011
15
0
18,510
So I plugged in my atx power connector cable (not sure if 5 or 6 pin, has 6 but 1 is hollow) in to one of the 2 slots on my mobo and it turns on for a few secs, switches off and smells like burning! At first I think the CPU has fried but unplugging everything I find the cable has a burnt socket and some of the wiring has melted!

I plugged another cable into the other slot and everything works fine now, just wondering if anyone could explain what I did so wrong!?

Thanks.

(mobo is a P5Q PRO if that helps)