So I take it we all have built PCs and have had moments that we're all no to proud of. Screw ups, mistakes, blunders, and just plain foolness is par for the course.
So in short, what's your worst screw up when you've built a PC?
Mine was the time I built a P2 system. First ever build. I had everything set right, CPU, GFX card, sound card, jumpers set right, everything was good to go. I just forgot to use the standoffs when I was attaching the board to the case. Every contact on the board was touching the metal case. Needless to say, I fried both the board and the CPU.
First time I was ever greeted by the magical blue smoke. Not my last.

EDIT: Think of this as therapy for computer users.
EDIT 2: Now accepting all sorts of PC trouble woes, that includes Macs, iPods, hard drives, RAM, spilled liquids (including but not limited to beer, wine, coffee, water), VIA, electrical arcs and just about anything you didn't plan for. I'm just not taking any tech support stories. That's a category onto itself.
EDIT 3: Now added the Murphy's Laws on computing. To all, keep your posts relatively on the topic about tech woes or computing destruction. Don't spam the tread.

Murphy's Computing Laws Treat 'Em as Gospel Folks...

1. When computing, whatever happens, behave as though you meant it to happen.
2. When you get to the point where you really understand your computer, it's probably obsolete.
3. The first place to look for information is in the section of the manual where you least expect to find it.
4. When the going gets tough, upgrade.
5. For every action, there is an equal and opposite malfunction.
6. To err is human . . . to blame your computer for your mistakes is even more human, it's downright natural.
7. He who laughs last probably made a back-up.
8. If at first you do not succeed, blame your computer.
9. A complex system that does not work is invariably found to have evolved from a simpler system that worked just fine.
10. The number one cause of computer problems is computer solutions.
11. A computer program will always do what you tell it to do, but rarely what you want to do.
12. A computer makes as many mistakes in two seconds as 20 men working 20 years make.
13. The computer will work perfectly at the repair shop.
14. Never test an error condition you don't know how to handle.
And Finally:
15. Computers are unreliable, but humans are even more unreliable. Any system which depends on human reliability is unreliable.
These are our 15 Commandments. Live by them or be prepared to put your head thru a wall in fustration.

Thanks to everyone for the submissions and please keep them coming.
Big thanks to Wusy, verndewd, greenjelly, ches111, Clue69Less, StrangeStranger, angry_ducky and exit2dos and whoever my feeble mind fails to thank for responding to stories, providing feedback and giving topics for discussions.

this comp was my first buuld and it went pretty bad but that is just my way.

even though i had done a few upgrades without harm i still knew i would make mistakes.

my main mistake was that after putting it all together convinced i had screwed up was that i chose not to buy a new power cord but to use an old one i ahd lying around. needless to say, unbeknown to me it was faulty so when i tried to switch on my comp it wouldn't. of course i then took the whole thing apart and went back to square one where finally i tried the cord from my old comp and suddenly i had power.

of course i had to then put the thing back together and put new thermal paste on the cpu and hope all that fiddling hadn't screwed a component or something. luckily it is still working as we speak over 6 months down the line so touchwood it ain't gonna die soon.

F Lock. Didn't know there was one when trying to accept Microsoft windows xp license via F8.

Atleast yours worked. In my youthful exuberance and stupidity, I wouldn't accept any help from my dad or listen to what he had to say. I ended up having to pay for the new components. Try paying for a P2 back in 1997 when you're only 9...
Lesson was learned well. He only made me pay $50 though so its all good.

This wasn't a real bad one, but my first build I didn't turn the PSU rocker on the back from "0" to "-". I spent two days troubleshooting, disassembled everything, and took the PSU to CompUSA to have it tested.

Here's the real kicker: CompUSA's technician said the PSU was faulty. On my way home, however, I noticed that the rocker switch was on the "off" position and realized that the dummy at CompUSA tested it that way. I put the PSU back into the computer, flipped it to the "on" position and everything booted up. What a stupid mistake.

lol, thats pretty much like me except before taking everything out i should've tested it with the working power cord and saved myself alot of hassle.

I used to work in a pc repair shop, one of the biggest blunders I was involved in happened at this shop. I had just finished the burn in process on a pc I had just built. The burn in was a program we had that tested the pc for stability for 48 hours. I noticed the temps were a little high so i opened the case up to make sure all the fans were on. Now mind you im ina pc repair/building shop. We have canned air, cleaning spray, thermal paste all within arms reach. So i open her up to check the fans and notice a blob of dust on the cpu heatsink. So i reach over and grab what i thought was the canned air. Wrong! It was the highly flamable cleaning spray. Needless to say that pc went up like the hindenburg.

would have loved to have seen that. still i think theres a few dell laptops that could also perform pyrotechnics.

Battery acid flames pale in comparison to the wonder and delight that comes from watching gases ignite. 8)

With respect to PC builds specifically, I've been lucky, no horror stories. Now if you go back to the days of my first computer experience (PDP-8A) I've had a couple of little problems... If you ever find yourself designing printed circuit boards - double and triple check all of your circuit logic before you make the board. Then, when the board is in hand, beep out all of the power traces and re-check the schematics for the chips and other components you're about to solder on.

A good Burr Brown A/D converter is a horrible thing to waste...

Well this isn’t a hardware screw up per say but I got sick of looking at my silver case so I spray panted it, not so well I might add. but it is no longer silver.

I guess I have been faily good at not frying any components (well sort of Ill explain later) the worst thing I have done is bent a pin on a cpu. not a prob to fix though.

now as for the not frying stuff I did eff up a psu once but it was one of those intentional "accidents." well in my computer hardware class I had in high school I didn’t like the teacher so well and I kinda wanted to cause him some problems. so I took an old at power supply crossed a power with a ground, poof went the psu and pop went a breaker. The whole class lost power and we had to call down a guy so he could reset the breaker, most defiantly worth it.

Me worst misstake is not that bad, though it was on the last pc I put toghter (4 or 5 in the order). All when ok until I was to add the silence material on the inside of the chassie, is was sticky as hell and the first I did was the easy removable door and then the rest of the case, which in it self was tricky, and when I'm done I try to put back the door and realize I put the silence material so the door doesn't fit anymore =(

My worst to date was when I replaced a motherboard that have a Nvidia chipset with one that was from Via. I couldn't quite read some of the wiring instructions, so I hooked up the wires the same as on the Nvidia. It lasted as long as the opening screen, then nothing. Getting out a magnifying glass, I found that the wires were supposed to be hooked up differently. Live and learn, and see a doctor for some new, more powerful reading glasses. I've also used only Nvidia shipsets since.

My worst mistake was not that I forgot to connect the power dongle to my 6800 video card, but when I turned the computer on and got that warning tone and I quickly pulled off the side cover and plugged it in with the power still on. My video card was a goner.

hball

When I used to work at a system builder, we always had the QA people at the end of our line to test every computer. I don't know if this was my mistake (Probably was since I was one of the few people who actually installed the CPU) but one of the QA people tested a computer, and a "jet of flame" shot up form where the CPU was... shrug... Everybody could use a jet of flame once in a while.

Probably giving away my age with this one, but when I built my first PC it was an XT and the case I bought didn't have the power connections on the switch hooked up. So I hooked them up myself reading the color diagram on the back of the switch, bl for blue and b for black and w for white, etc. well I got the blue and black on the wrong connectors, which equals blue sparks and black smoke. And if anyone remembers the old power supplies before ATX were hot at the switch with 110 volts AC.

I got lucky though and only fried the switch and power supply, Mobo, CPU were good though.

Back then I really thought I had something with 10 Mhz Intel 8088 CPU, 640 Kbytes RAM, CGA graphics, two 5 1/4 floppies and a big 10 Megabyte hard disk. Anybody here remember what a Turbo button was?

other than jamming in a molex upsidedown and cooking a hardrrive while the power was on, thats it. My bro though tried to put a thermal sensor between the cpu and the heatsink, POOF!! The heatsink still has the burnmark on it to this day.

One more:
This wasn't a build, but I was working on Computers at work once and my boss would jump my case if I unplugged a computer while working on it, because he said it wasn't grounded and static discharge would have nowhere to go if the cable was unplugged. I was upgrading NICs in some of the PC's one day and the first one I plugged in caused the Wake on LAN (WOL) to turn on the machine as soon as I plugged the new card in. No damage but it sure startled me and I said, 'screw the boss' and unplugged every computer I replaced NICs in that day.