What are some mistakes you made while building a computer?

I hope to build mine before the end of july.

I purchased everything hoping graphics cards would be available by the time I finished.

Not changing the stock thermal paste on the Wraith Max cooler that came with my 3900x. It's not bad performance wise but the darn thing is like glue. I remember while I was assembling my PC outside the case I first put it on the cpu, and I'm not kidding, I could lift the motherboard by holding just the cooler which wasn't even secured unsecured. And when when I got an aftermarket cooler, removing the wraith was one of the worst experiences I had while building a PC, I was sure to yank the processor from the socket but thank god I didn't.

My first ever mistakes where when I bent pins on a FX-8120 had to return to AMD since I ripped off a sticker from the box it came in Newegg would not take it back. 

Red_Infern013
Not changing the stock thermal paste on the Wraith Max cooler that came with my 3900x. It's not bad performance wise but the darn thing is like glue. I remember while I was assembling my PC outside the case I first put it on the cpu, and I'm not kidding, I could lift the motherboard by holding just the cooler which wasn't even secured unsecured. And when when I got an aftermarket cooler, removing the wraith was one of the worst experiences I had while building a PC, I was sure to yank the processor from the socket but thank god I didn't.

And this. Thermal paste related stuff always makes my hands shake

Only thing I can think of is trying to put 10 Corsair ql120s in my lian li 011XL. Cable management was a nightmare. Return the Corsair fans and got lian li uni fans and they were super easy to cable manage.

Flaretrail

Klaus H
I purchased everything hoping graphics cards would be available by the time I finished.

This is making me very wary of buying any other parts, even at good prices, before I secure a GPU... Found a ryzen 5 5600x for example, for 280€, but I just cant pull the trigger yet

You can try and find an old second hand GPU for a decent price. Old GPUs still give a decent performance that surpasses the most recent iGPUs. Something like the gtx 960 4gb is infinitely better than the gt 1030 and you can buy it for around 150 usd on ebay. The gtx 600 and 700 series are still good too in my opinion and you'll find them even cheaper.

Losing one of those old little connectors for jumper pins back around the end of the previous millennia was probably the most significant to date, but I'm frustrated by something else this build...
 
I have health issues, including with my joints, I bought a huge case to make sure I wouldn't have to fiddle with things requiring much manual dexterity. Turns out the case I bought has an entirely separated PSU cage that expects superhuman flexibility from your hands to get the cords through the right openings. I'm going to have to have someone come and help me assemble the thing and it's driving me up the wall.

My biggest mistake was not connecting my 2 video cards in my custom loop, then turning on the pump heard spalshing and then puff, good news is I only fried the power supply both cards were fine after drying off.

I made the mistake of using a modular cable from one brand of power supply with a different brand.  Yeah, they didn't pin out the same.  Fortunately all it fried was one of my hard drives I store movies on.

Do not forget about the IO shield.
You should also not cheap out on the power supply. I speak from experience. Go with a gold efficiency standard single rail PSU.
 

Deciding to put in an internal DVD burner even though a barely use it and it takes up space that would be otherwise used for better cable management. Not going with the 750W PSU when I originally had the chance. I remember on a very first build I put the power pin in the spot for the reset switch and visa versa then wondered why the PC didn't power on. Needless to say I did far more troubleshooting then I should have. I recall putting in a 32bit copy of Windows 7 into someone else's PC I built and wondering why it wasn't working with 8Gb of memory.

Probably my dumbest, and most recent mistake, was not having a replacement res tube screwed down tight onto the base.  Started filling and realized I had liquid on the counter.  Now that doesn't necessarily sound too bad, until you understand that it wasn't the standard coolant.  It was the Mayhem's Blitz kit solution.  Oops.  I had no idea if it was going to eat paint off or otherwise damage stuff.  Plus I had a rad with fans right under it so I spent a lot of time blowing that out with air. 
 
Before that, on one of my other builds I do remember forgetting the I/O shield.  Yep, I was "that guy" lol.

I would have to second the thermal paste on the AMD cpu coolers
Over the years I have had to remove a ton.. and that stuff really is like glue.
One time I yanked so far on it the CPU pulled out of the socket still stuck to the heatsink. Luckly the lock-in level pulled up otherwise the pins would have been gone.
Still had to use a heat gun and screw driver to pry it off.
 
The more common mistakes I have seen others doing (used to repair them)
1. front header pins plugged in to the wrong jumpers
2. 4pin PCIE and Mobo power switched around
3. Not realizing the backplate hasnt been installed until the whole pc is assembled.
4. not using motherboard stand offs on the tray and frying the mobo
5. installing the fans wrong on the AIO coolers and having them push air back into the case (as well as all the other fans being intake too.
 
6.  The best one ever.. is one of the guys I worked with did this not once, but twice..
Plugged the floppy power into a fan header on the mobo and blew a hole in the motherboard. Heard a loud "boom"  and some cursing.. he got a new board and did it again.. I went to take a look and couldn't stop laughing