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.
back around 2013 I was working on a school project, did the ol’ get out my laptop without putting on my glasses bit, and ran “rm -rf” on the wrong folder because the characters sorta looked similar to my farsighted eyes.
Since I didn’t make backups, that was a few weeks of work down the drain.