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.
In my case it didn’t even need sudo to ruin my day. I wanted to delete a temporary directory in home by typing
rm -r ~/ tmp. See how a space snuck in between the slash and tmp? Yeah, great day that was.