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.
Not Linux, but OpenBSD. I got a sun ultra 5 for free so I decided to make a router out of it. After some research OpenBSD looked like the best option. I bought a pf book and started writing configs. After about a week I had a really nice router that did exactly what I asked it. This was back in the early days of xbox360 so getting all of the port forwarding right was kind of a pain since we had three of them connected in our apartment along with all of the computers. Then the harddrive crashed and I hadn’t made any backups. That was a lot of work down the drain.