About four years ago I took a hard drive out of my computer and turned the thing on when the drive was not in the horizontal position. Result: lots of smoke and cursing. It still worked though...
Blew the PSU on my mom's computer from work by switching it from 240V to 115V - I thought it wasn't on and decided to try this little red switch on the back to see what it did. It broke it. My parents took the computer to my mom's work and pretended it didn't turn on properly...
On an old computer about 7 years ago (a BBC Micro - a "classic" British computer), with mobo-mounted sockets, broke one of the pins in the floppy connector off. I took it into school and attempted to desolder the socket and put a new one on - of course it never worked again and it went away to computer heaven.
Bent a pin in a monitor connector at work

. Fortunately someone else bent it back...
Broke lots of tape recorders (ah, the days when floppies were rare...)
Cracked a monitor screen on a little monochrome Phillips 12" - still p*ssed off about this one.
Broke the keyboard connector on my Atari ST. Blu-tak repaired this
Put an Abit KT7-RAID motherboard in it
Broke the connector on a SCSI card. Whole card replaced for free
Lots of scratches and dents...
Etc, etc. I am the living, breathing inspiration for Mr Bean...