Thought this might be a fun topic for a few light hearted laughs having people share some of there stupid mistakes they made while building a PC.  Anyone that been building PC's for years made some stupid mistake a few times in there life at least.  I built my first PC 21 years ago.

For me there two stupid mistakes I made that come in mind, one resulting in damaging parts and another coming close to harming my self (luckily I moved my face away in time)

1) I had a PC I built in early 2000's and was doing some upgrades on it.  This was back in the day when it was popular to have multiple 12v rails on PSU's.  That fad mostly died out now and most PSU's got a single 12V rail these days.  I open the case to upgrade the parts and notice I had made what looked like some odd choices connecting cables to parts where there was the same cable type closer to the part.  I upgraded the parts and when I reconnected the cables I just use the cables that was closest and made cable management the easiest.  Turn out I had a very good reason for connecting it the way I did when I originally built it and this time I had put all the 12V intensive parts on a single 12V rail which could not handle that kind of load.  When I hit the power button I heard a loud pop and black smoke came out of my PC.

luckily this mistake only killed my PSU and everything else ended up being fine but I was very nervous I took other parts out with it and I cant express how relieve I was when I connected a new PSU and everything worked.  That PC had a AMD FX-55 and a Geforece 6800 Ultra in it and for those not familiar with those parts they was very expensive CPU and GPU(for it time, GPU prices have sky rocketed since then).   I was a college student at the time and saved up for awhile for those parts so if either of those had went I would of been skewed.

2) I thought it would be fun doing a fill custom loop in a itx case (both cpu and GPU on the loop).  It was a huge pain getting everything setup but I did manage and had it running for about 8 months no problem.  Then it started to turn off within 15 seconds of booting.  After the 3rd time I open the case up and turned on computer while sticking my head in to see if I can hear if the pump was working.  This was very stupid on my part especially since this was possibly 4th time now am starting a water cooled computer with a broken pump.

I heard hissing noise and I pulled my head back just in time.  A second later I would of gotten a face fill of steam.  The water over my GPU actually boiled and the steam escape the joints of the fittings attach to GPU.

No harm done but came very close to burning my face.  

Cleaned my case about 15 years ago and didn't know at the time that the fans should be disconnected before cleaning them, only switched off the main power and pulled the plug. but the rotations of the fans caused by my cleaning built up a big amount of static energy in the board, and when I tried to turn on the PC afterwards I pretty much fried the motherboard. That was the end for the 233 Pentium MMX in my secondary PC...

Bought a HP desktop or my first computer.

Didn't even know you could build your own computers lol.

Was overpriced, compact and hard to upgrade parts inside.

I said "If anything goes wrong, feel free to call me" when I finished fixing my uncles computer.

Not sure if it qualifies as a mistake but I had a PSU capacitor spectacularly exploding and filling the room with stinky steam after I thought the PSU had been fixed.

Haha, when I was really into PC's I started a PC build business. Made about 7 or so, and the huge mistake of saying call me if you need any help.

Needless to say I've never made a computer for anyone else since then. What a nightmare of parents being like, "I keep getting viruses"

Replacing a GPU cooler with a third party one and not securing it properly. It literally came loose and fell off while being transported. And I didn't realise until my PC shut down while gaming at my friends house. My Radeon 9800 Pro fried with it. Sad times :<

I also had a problem once of using an underpowered PSU for the parts I'd included.

One of the scariest things you'll ever have happen is a pop and black smoke. Thought I killed the whole pc

Lol yeah, happened to me too.

My neighbor had a game disc with a virus on it. He also had an antivirus who totally locked down the entire OS and thus PC when he found said virus. So I had to juggle into the BIOS to get it to boot from a CD (was the late 90's, booting was generally either HDD or Diskette), format the entire PC and reinstall everything except the game. Took me about 8 hours to do so. What's the very first thing he does afterwards? Insert the infected game disc again and tries to play it!

AAAAAAAAAAAAARRRRRRGGGGHHHHHHHHHH!

I completely fucked the pins on a 200€ ITX mainboard. Immediately bought the exact board again and threw the other in the trash to destroy the evidence.