Which mistakes did you wish you were warned about ?

Hello ! I'm building my very first PC and a kind soul helped me assemble a part list : (France, Gaming and ligh editing, 1080p but perhaps I'll upgrade in the future) (https://fr.pcpartpicker.com/list/8JbzRK)

I've watched a few videos and i'm starting to feel relatively confident to assemble this next week, but I'd love to know, when you built your first (or not !) PC, which mistakes did you make that you wish you would've thought about beforehand ? There's a lot of tutorials and tips on the internet, but experience is always valuable, so I'd love to know about your fuck ups so that I may try to avoid them !