Maybe 20 years ago... As a student, the school had an email server that allowed rules to be set. You could set an email to be sent as a result of another email.
IT were not stupid though, and set a series of rules:
1. You cannot have a rule trigger to email yourself.
2. You cannot reply to an email triggered by a rule.
3. You have ~50MB max of emails (which was a lot at the time).
Playing around one lunch, my friend had setup a "not in office" automated reply, I setup a rule to reply to any emails within our domain with a "not in office", but put their name in TO, CC and BCC. It turns out that this caused rule #2 not to trigger. After setting up the same rule on my friend's email, and sending a single email, the emails fired approximately one every 30 seconds.
A few hours later we returned to our email boxes to realise that there were thousands and thousands of emails. At some point we triggered rule #3, which in turn sent an email "out of space", with a small embedded school logo. Each one of these emails triggered our email rule, which in turn triggered an email "could not send message", again with an embedded logo. We desperately tried to delete all of the emails, but it just made way for more emails. We eventually had to abandon our efforts to delete the emails, and went to class.
About an hour later, the email server failed. Several hours later all domain logins failed. It turned out that logins were also run on the email server.
The events were then (from what I was told by IT):
* Students could not save their work to their network directory.
* New students could not login.
* Teachers could not login to take registers or use the SMART white boards.
* IT try to login to the server, failure.
* IT try to reboot the server, failure.
* IT take the server apart and attempt to mount the disk - for whatever reason, also failure.
* IT rebuild the entire server software.
* IT try to restore data from a previous backup, failure. Apparently the backup did not complete.
* IT are forced to recover from a working backup from two weeks previous.
All from one little email rule. I was banned from using all computers for 6 months. When I finally did get access, there was a screen in the IT office that would show my display at all times when logged in. Sometimes IT would wiggle my mouse to remind me that they were there, and sometimes I would open up Notepad and chat to them.
P.S. Something happened on the IT system a year later, and they saw I was logged in. They ran to my class, burst through the door, screamed by username and dragged me away from the keyboard. My teacher was in quite some shock, and then even more shocked to learn that I had caused the outage about a year earlier.