Apparently, we have a report of the worlds first DoS attack (http://www.platohistory.org/blog/2010/02/perhaps-the-first-denial-of-service-attack.html).  In 1974, at the University of Illinois Urbana-Champaign, in their CERL, the Computer-based Education Research Laboratory, they used the TUTOR language/system on PLATO.  The Author (admin/super user) could issue the EXT command allowing you to connect external devices, if a device was not hooked up, the machine would be locked out.  The default option was that the command could be issued to remote terminals.  Well, a small program, and 31 pissed off users later, the first DoS attack existed.  The "hacker" issued the command a few more times across the system, but it was quickly patched by some diligent admins.

This was of course back when we still had hippies running our networks.  Free love, trust everyone, the smell of nag champa, playing hacky sack in the server room, and all of that.