Just having acquired basic Unix skills, I tried to show a friend, with whom I was sharing the account, a stupid joke, involving a shell command. So I did "cat > name" and typed a simple echo command or something like that. However, "name" was the file where my friend had stored his nearly finalized essay in nroff (we're talking 1985 here) that was printing at that very moment. We could never recover the file, but it completed printing fine...
Another f*ck up, much later, was removing (or perhaps moving, I can't remember) an essential dynamic library, perhaps even libc. Suddenly all commands quit working. It was a headless box, so the only thing I could do was try to repair it using commands from /bin, since these still were functioning (good thing they don't rely on dynamic libraries)! Had I logged out or lost the connection, the box would have been rendered completely useless.
Another stupidity was installing DEC Ultrix (from tape!) on a DEC-station and answering "yes" to the question whether I really didn't want to change anything to my set-up. I thought it meant the current configuration of the work station, but in reality it referred to the default values just shown. So the installation procedure happily started to repartition my drive, losing 300Mb of files.