I recently told this story about a former colleague who managed to create a mail flood between his inbox and mine, but here it goes again.
Whenever I or the other senior tech on my team went on vacation we'd set up a regular OoO response and, for a select number of addresses, a supplemental forwarding rule to the other one of us who was there, just to make sure requests from higher ups were properly followed up. One day I went on vacation and set the rules and, during that period, my colleague had to take a day off so did the usual. Unfortunately the forwarding rules weren't set to just forward mail from those contacts, but also when they were CC'd... so when the first email form someone on the list was sent to his address (or mine, really don't remember who first triggered the glitch), mails started being forwarded from one account to the other until it froze the Exchange server). He got a bit of an earful the next day, but nothing of consequence, and learned not to do it again (which is always good :) )