I meant to dd a partition to tape... and ended up doing the reverse on our fileserver. Tape is slow, so I caught it after a few kilobytes.
Somehow I managed to salvage the situation using the man pages for the filesystem, the C compiler, and making sure I did not reboot. Really don't remember much about that late night...
But this was one of my first. Years ago, making boot floppies for a physics lab where I was reinstalling all the servers:
I meant: dd if=/dev/zero of=/dev/fd0
I did: dd if=/dev/zero of=/def/hda
Oops. Bye, partition table.
(Always double-check everything you type as root.)