I was on a DEC training course in Reading, VMS Administration. I'd been a VAX programmer for a few years, but it was decided I should also get some mad admin skillz as well...
The first day of the course covered the basics, which I already knew - before joining my employer, I'd had a summer job working for an oil exploration company, working in administration and FORTRAN programming.
We then got onto the force logoff part of the day and, as expected, people were randomly logging the others off the system - it was a training machine, dedicated to our classroom.
I had a brainwave, wrote a few lines of DCL and ran it... It produced a list of logged in users and, if the username wasn't mine, it logged the user out. At the end of the run, it then re-submitted itself as a batch job, so it ran constantly. Oh, what a laugh...
The best bit was, whilst you were logging on, you had an entry with the username <LOGIN> and a process ID, so, everybody was being killed off, before they could even log on! A hoot!
Then, idiot that I am, I accidentally logged myself out! Not so funny now!
I couldn't log back in. Nobody could log in. The instructor took us into the backroom behind the classroom, sat at the console and started to logon directly at the console... BLAM! Session killed. Login, session killed, login, session killed. In the end, we had to perform an emergency stop and reboot the server.
The instructor was very good about it. I had no come back over the incident.