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

I didn't ground myself and fried a brand new motherboard. sigh I haven;t built one since.

Early adoption of Vista BitLocker Drive Encryption was my team's biggest mistake.

One of the officers was totally locked out of his brand new laptop. At the time, there was no known backdoor. We booted off our Ultimate Boot Disks, but the data on the drive was not accessible. Ended up reimaging the laptop, then restoring from the most recent online backup. Stored Recovery Keys with their admins, from that day forward.

My first build was smooth . The second one was problematic . I forgot the 8-pin connector. I freaked out a little then retraced my steps and find the mistake.

I forgot to plug the the TEC (Thermal Electric Cooler) power connector (from my V10) into the power supply.
So I had been using it for a few days and had been overclocking...
And I was doing some video rendering and suddenly BLUE SCREEN.
Rebooted opened some temperature monitoring software, started rendering the video again, and saw the numbers raise.
Until they hit about 94c and then, again....... BLUE SCREEN.
A little while later I worked out the problem.
Luckily no damage :)

My worst error was when I was about to put the CPU in. I added some thermal paste to the back of the chip and was about to install it when I dropped the CPU onto the mobo. The paste went all over the CPU PGA's and bent the pins on the CPU :-( -