For me it is not recording credentials with the assumption I would simply remember them later, while having every opportunity to archive them before eventually forgetting. Also, not keeping detailed enough notes & photos of exactly how my hardware is attached.
I installed some library from sources on my working laptop, and it stopped booting lol. Had to change my laptop for the newer Thinkpad, because you cannot insert into working devices any flash drives to boot from and fix the system. It hasn’t cost me anything, but was pretty funny