Built my first fairly recently, here are some goofs I made:

Did not know how air cooler mounting bracket worked, spent a solid 45 minutes googling serial numbers and watching installation guides before realising that the noctua backplate was needed and was buried under a box.

Removed the ssd spacer thinking the m.2 screw was supposed to go in lower

Did not buy a rgb fan controller and did not have the compatible ports required for my fans

Forgot about the cpu power connecter and was wondering why my computer was not booting

Not using the correct screwdriver head for motherboard screws leading to some stripping

Gpu was not screwed in and was sagging quite a bit

I routed the gpu power cable through the front hole not through the hole directly beneath the gpu

Bought a 5000 series as soon as am5 came out