First PC disaster I ever had was with an old 900Mhz Celeron in a desktop...I'd start the PC and I couldn't figure out why it kept rebooting. I opened the case (mini atx) and saw that the PSU fan was dead....which caused my processor to melt to my motherboard
Second one was when I was messing in my new desktops case...I moved a PSU connector which made contact with my metal cause cuasing my pc to turn off. I though i fried my whole PC when it wouldn't reboot. But i waited a bit and it rebooted (yay!).