personal experience:
I was putting together a brand, spankin' new Pentium processor (the only one around, in the 486 age) into a serious killing machine, with double tower case (meant for two-mobo servers).
I plugged in the hard drives, and booted up the machine, leaving it open for any minor fixups.
It was due to this incident that I have learned NOT to play with case screws over the open machine. I dropped one of them, it came down on the HDD's open circuit board, and since the drive was angled (hanging outside by cables), the screw went sliding down the whole surface of the board, leaving sparks and smoke in a beautiful diagonal line until it came to the edge and fell to the floor.
Needless to say, drive was shot as surely as with a gun...
to make things more interesting, I was 15 years old, and my boss was sitting on a chair above me (I was sitting on the floor while putting the computer together). Add to the fact that the 40MB drive was full of local car-insurance company data... ahh, the look on his face, as he stood towering above me... not something I'm likely to forget
Almost equally nightmarish experience was with a co-worker of mine -- we were putting custom computer together (clone 386/486s) in the back of a computer shop. That day he had a bad streak of defective motherboards -- he'd put the computer together, but it wouldn't boot... after 3rd or 4th in a row, we were seriously teasing him. After fifth we were getting concerned.
After sixth, we realized the mobos were not, in fact factory defective -- it turned out he walked across the carpet leading to the tech room and forgot to ground himself (no fancy wrist straps in those days in that part of the world)... so he basically burned six perfectly fine motherboard... I'll leave it to reader's imagination to realize the hell that followed