mine was when i was building my 2nd PC ever, and when I was hooking it up and turned it on the first time, i fried the CPU fan pins on the motherboard.  
and the worst part was that this motherboard had the protection that would shut the computer off if the fan on the CPU fan port was not working correctly.  so basically, my motherboard was useless to me ...
until after hours and hours of searching through the manual and internet, i found out how to boot the mobo up w/this disabled, and went into the bios and disabled the setting for good.
there were 2 CPU fan controllers next to eachother, but if the CPU1 fan controller does not have a fan powered on it, it would not power on the computer.
btw, this was a soyo dragon+ board.