Singed pin on ATX connector of the board?

[bkspitfire]

hi everyone, i was repiecing a computer a friend borrowed, and as i was putting the atx power connector into the motherboard from the power supply i saw that one of the pins appeared to be singed slightly, like it was burnt for an instant. Does anyone know what could cause this, and if its safe to plug it in and still use the board? I remember it working when we powered down the unit when he returned it to me.

Any ideas? to give you an idea its the 4th pin down from the top, right side.

Thanks.