Back in the very early days of my career when I was still a phone monkey on a "help" desk, before we'd hit on the concept of having a single email address to contact the help desk on, our customers would contact each of us individually depending on who their pet help desk operator was (there were only two of us). One day, for a laugh, I decided to set up an autoforward rule so that any email that came in to me got forwarded on to my colleague for them to deal with.
It didn't take them long to realise what was going on, so in a moment of brilliance they decided to set up a similar rule to forward any email they received to me. It doesn't require a genius to work out what would happen next, and we certainly weren't geniuses. Within only a few minutes the exchange server fell over and our phones started ringing off the wall as our users started phoning in to complain that email was down. Of course we immediately escalated the issue to our second line support and in a flash of brilliance decided it might be best to delete those autoforward rules.
In our defence we did come clean to the manager of the team that looked after the server, but we waited until everything was back up and running and the fuss had died down. Luckily she saw the funny side....