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

I bought a brand new fancy asus p5k extreme motherboard and fitted an audio plug (front panel) onto a header without checking it was the right one. Immediatly the pc powers down nothing I could do could get it working again, I contacted the company I bought it from and told them they sold me a dead motherboard a got sent a replacement the next day! Ha! Needless to say I read the manual that time and put the plug on the right damn header!!!

I changed the voltage regulator without turning off the power.

It blew up literally in my face like a flashbang!

I had double vision for a very short while but one of my ear keep ringing for days!!!

I have not built a computer yet, I'm planning on building one soon though. I'll say what I did when I was rebuilding a computer for my first time. I did not know how hard you need to press the RAM in so I did not wait for the click. I was hearing beeping and I was wondering what it was, eventually I figured out it was the RAM was not in all the way. I'll never make that mistake again.

Hahaha never have built one, but I'm in the process, gonna see what people say here so they don't happen to me.

When building my first computer I installed a cheap aftermarket CPU cooler.

Little did I know that it was not clipped in (it wasn't screw, but that cheap-o push pin type) and after about 50 seconds my computer crashed.

Solution: bought a Zalman cooler.

The biggest mistake I made... Well, I custom built my computer, and when I bought all of the components online, I forgot to order a power supply =/ It wasn't a really big deal, I just had to order one and boom :) But I can't believe I forgot it in the first place :P

Proud to say i've burned up 4 motherboards by not squizzing all of the screws that hold the motherboard correctly so letting them get the power supply they needed :P

I forgot to connect the hard-drive, then I turned it on and it started making weird noises so I started freaking out, and ripped the plug out of the wall. All that just because I forgot to connect a wire.

I was lucky and nothing went wrong the times I built a system, well, there was an issue with the front USB once which was fixed without any trouble.

This is not really a build but it's a pretty stupid story all the same.

I decided to put all my work onto my new computer because it had a new drive and I figured there would be less chance to loose my precious files because the old drive was getting old. So I copped them to the new computer and wiped the old one ready to do something else with it.

I left the pc on with the files open for about 35 minutes to make a call and have a cup of tea. I sat back down and went into autopilot, totally forgetting that i'd already deleted the files off the old computer, I assumed this was them and promptly deleted the backup. Boom Baby. This 5 years of stuff included such things as some of my favorite photos and websites i'd made and all my university and college work plus a bunch of other stuff.

I always have the recycle bin turned off so I had one of those jaw dropping moments.

The funny side is that I always tell other people to backup because no matter how pro you are, you can still make silly mistakes.