FYI, I don't know if this is a common problem or not, but the other day, my wife was using our computer with a Asus p4p800s-x Mobo and it burst into flames! I've just pulled everything out of it and it looks like the fire was at the edge, under the lower PCI port where the USB connectors are.
We are very lucky that this happened when she could cut all of the power to the computer and it didn't catch fire in the middle of the night, or when we were all away.
The back of the MOBO was melted to the wire and soot was all over everything. We have been using this machine for several years without any problem.
A power connector that has come loose for whatever reason may become too hot due to high contact resistance and cause a burnout plus what tecmo34 said.
I do not think this is a common problem.
If you keep connections tight, inside of PC case free of dust, all components free of short circuits, then you are not likely to experience fire. Defective design of parts and notebook batteries excluded.
Dust was not an issue. At this point, I suspect it was probably due to me miswiring the USB/Firewire conectors. It could be possible that the "front case" firewire connector was connected to the USB pins or visaversa.
The thing that amazed me was that it actually caught on fire -- with flames and all!