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.
Encrypting a drive with Linux, then encrypting a VM within Linux with my Bitcoin wallet information in it, which I was gifted 5 bitcoin before it was popular and just forgot about it. I was 13 at the time and didn’t know what I was doing. Lost all my passwords, or I might have even just wiped my entire drive. Got a pile of hard drives to go through and see which one has Linux on it, but that’s only the first step.