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.
Probably trying to share a Stream drive between Linux and Windows. Trying to run games from NTFS just didn’t work and resulted in all kinds of weird issues. I was close to giving up on Linux but after I switched to an ext3 partition things just started working :|