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.
Hibernating my computer and then forgetting about it and booting into a different OS (Fedora Silverblue) on the same partition (BTRFS subvolume stuff). AND THEN TRYING TO RESUME THE HIBERNATED OS (Arch btw).
my filesystem was pretty much unrecoverable and it was my fault