I am really skittish about letting computers run overnight unless they were designed to, like servers.
when I was much younger, one day I walked out of the room and the power supply had let loose. it shot sparks up from the fan vent (power supply was on bottom), and the ribbons, ATX wires, etc were laying on the power supply. it caught the wires, and the tower had JUST started to catch on fire. Luckily I heard the POW and ran over there to figure out what happened, and saw smoke.
At that point I had to rush the PC out of there and throw it in the utility sink (living in the basement at the time) and douse it with water.
Fun times. Ever since then, If I am not home or go to bed, the PC gets turned off...
granted thats the only time I have ever seen that happen as far as catching the PC on fire. ive seen many power supply blowouts, but usually the sparks are contained and dont drop hot metal on the wires. But this was around 2002 or so.