The graphics card caught fire. Fortunately, I was at my desk at the time and couldn't help but notice the heavy smoke boiling out of the machine. I pulled the plug pronto which halted the fire, and put the machine out on the concrete patio till it stopped smoking. Curious, I plugged it in again and the fire started again. REmoving the cover showed the graphics card and a section of the mobo was burned to a crisp.

I had to wash the floor, walls, everything in my office to get the stink out.

Replaced the mobo and graphics card and I was back in business, but this time I bought some sheet steel and set the computer on that, which hopefully would buy some time to get the fire out.

The machine had a metal case, which I'm sure greatly slowed down the spread of the fire. So for me from now on, it's metal cases all the way, baby.

I had, from what I can tell, the 3.3v power Button wire met and catch fire, no idea how and it sounds very unlikely but the computer worked once the fire was put out - I just had to start it with a screw driver.

I've seen the remains of a computer in a school, after the building was struck by lightning overnight. Only that one computer was affected, and inside it was mostly black dust.

I would guess something shorted in the power supply or power bus.