I've been fortunate to never really cause a major meltdown, but I was witness to a decent snafu back in my college days.
Our school had only two servers at the time, and among the various admins, only one guy was common to both machines. Email addresses were server specific, so there was no common, main address for this admin. His bright idea was to set each of his server accounts to forward mail to the other, so that no matter where he was working, he'd get notice if someone on the other server had a problem. At the time, our system popped up a notification line whenever you received email, and you had to clear the notification before proceeding with anything else.
So, Steve, our admin in question, sets up his forwarding, and sends a test message to his other account. Notification of an email to his present login popped up almost immediately. He clears it, and is frozen by another notice. Then another. After about five, he realizes exactly what he's done, then kills the computer, since he can't get enough time in between notices to log out. Before he can make it to either server room, both machines die, all space on their drives full from his email test message. It took him about a day to fix, if only because he couldn't use either of his logins because of the notices, and the fact he had to clean both machines, located across campus from each other, at exactly the same time.