Long ago I was trying to solve some thorny problem that I now forget and I thought it might be a good idea to uninstall and reinstall glibc. I knew it was a bad idea but I was at the point where I was past caring and figured why not see what happens. Turns out, in order to uninstall glibc there's a confirmation prompt, and instead of just 'y' or 'n' you have to type a whole sentence that's something like "yes, I understand this is a really bad idea". Well, I did that, and it was a really bad idea, the system was, unsurprisingly, basically unusable after that.
Would have used /media but was thinking, if, say I forgot to unmount it or someone looked at a disk free or whatever, that it would be obvious that it was there temporarily.
Obviously that was incorrect, but the reasoning was, I think, sound.
I did something similar. I moved some files around between a university computer and an SFTP mount (mounted through the ubuntu UI). When I leave those computers, I always execute "rm -rf ~" because they are restored to some image every time they boot anyway and I'd rather not leave anything personal behind. It was only when I started seeing "access denied" in my terminal that I realised that ubuntu had mounted the entire server on the other end in some hidden directory in my home folder, and that I was deleting every file on it for which I had write permissions. Luckily a quick CTRL+C saved my own files but I'm not sure the same could be said about a few students who were unlucky and had "loose" permissions in their home directories.
Forgot about tmpwatch, a default entry in the RHEL cron table to clear out old temp files.
4AM the next morning, recursive deletion on anything wiuth a change time older than n days.