What's the biggest mistake you've made when building a computer?

I had the motherboard ontop of the case's frame to test it out before i mounted it....... turned it on.... cpu and motherboard fried because the circuitry underneath was touching the metal frame.

This is where I lose significant "geek cred" points by admitting in a public forum that I have never, ever, actually homebuilt my own computer. I'm up to the task/challenge and I know many people who have done it, but for some reason I have never actually made it a project of mine to build my own system.

Bought the innards of two exact same laptops. On both, the screen was missing. One was minus the RAM, hard drive, LCD, etc, etc (everything but the mainboard). The other had a fried mainboard. Punched a couple holes in my desktop case and started to improvise, etc... Wanted to convert it to a desktop... Noticed the mainboard had capacity for another stick of RAM, so went online to buy a stick of what I thought was DDR RAM SODIMM. Turns out it was DDR2, when I got it I tried to force it in and broke the mainboard ;)

i forget the 8-pin connector on the motherboard.

Well after 3 days i actually found out that it was that. By that time i already bought a new power supply since i thought that was the problem. But the keyboard was hole new to me

Yeah, when i first started i didnt put the RAM sticks in properly, it started beeping when i turned it on and started freaking out, but quickly figured it out. :D

I had the PSU turned off while trying to boot up the computer. That didn't work too well...

humm i have not done much building but i would have to say i set both hard drives on the same IDE cable to master...

My Biggest mistake making a computer was plugging one of the front panel wires in backwards. The computer didn't care for that to much =D

I've never actually built my own computer. But I would and probably I would fry something up the first time :p

I didn't connect the power to the CPU, didn't realize... spent nearly an hour checking all other connections the mobo. i also brought out the power meter and checked the power supply wiring hehe I was relieved when i found the cable dangling down.