I was working at a large company, and an external party that pretty much everyone in the industry was working with, sent out a regular blast email to all its users about the industry. Unfortunately, they CC’d everyone. What ensued was an infinite loop of out-of-office replies being sent to everyone. I came back from lunch and I had over 500 emails in my inbox, and they kept coming. IT had to step in to put an end to it. Pretty funny to watch in real time.