New system, fried mobo, Please help.

I've built a system or two in the past but each time everything has worked the first time.

I'm building a new system for my mom's fiance, nothing top end but a fairly decent system. Problem is, when I put everything together and tried to turn it on, the only thing that happened is the LEDS / Fans kicked on, and immediately turned back off.

I spent some time rechecking wires from the PSU and everything seemed to be in place correctly, I turned it back on and the same thing happened. A little baffled at this point I proceeded to remove everything but the barebones necessities.

This time when I turned it on, it turned on again, turned right back off, same as the previous two attempts. I unplugged the entire system from the wall and went to the bathroom and got a drink. When i came back I AGAIN checked the wires, nothing out of place, nothing missing. When I plugged it into the wall to give it another go, before I even had a chance to push the power switch, and small stream of smoke billowed up from the mobo and a small orange glow was visible. I immediately pulled the plug and repackaged the mobo to RMA it.

My main question is, before I start attempting to redo this with the new mobo, is there something else that could have caused this to happen that I may not be aware of. I want to obviously avoid the same outcome.

Could it be something with the PSU?

Hopefully someone a little more experienced than I can help me shed some light on the problem.