I've cooked more of my own stuff than I can remember....
the WORST thing I broke though was when I was a hardware monkey, and I was trying to fix someone's Pentium 1 machine. This was about 4-5 years ago. I thought it'd be a bright idea to flash the BIOS (machine had some REAL weird stuff going on)... and you guessed it, the mobo wouldn't boot after that... I tried hotswapping BIOS chips, but no luck....
P1 mobos are bloody hard to find... it was an AT machine, and the only compatible board we could find was ATX, so they had to get a new case and everything....