Mine:
I installed a new wireless card, I decided not to put the screw in figuring that something would go wrong with my luck as ususal and I'd have to start trying different PCI slots.
I boot my computer up, everything's fine so I decide to put the screw in. Being the clumsy oaf I am, I drop the screw and it starts rolling around on my sound card and motherboard. All of a sudden my computer shuts off.
Power button... nothing. Tried resetting via power bar... nothing. Then finally I left the power off for a few minutes and then turned it on again, it worked, apparently my motherboard had some capacitance left in it so it just needed a reset.
My computer turns on.. everything seems fine.. but Linux wont recognize my sound card... i boot into Windows... nothing.
Tried all sorts of stuff, I blew my sound card that day
Another stupid thing I did: with my old PC, I noticed my PC was really dusty so I decided to get rid of the dust a bit. Instead of using the conventional means of getting rid of the dust, I decided to use the nossle on my vaccum cleaner.
So... I started sucking up the dust all around my PC... and it was getting pretty tidy, I was impressed with myself for thinking of such a great idea! Me being clumsy again... I slipped a bit and the nossle hit some of the capacitors on the motherboard loose and sucked them up.... DOH
Luckily I had just bought the motherboard and the guy at the computer store I bought it from was a real jerk and an idiot so I just exchanged it