Brings back memories
Many years back whilst working for another company we arrived in the office one Monday morning to find the mail servers on the floor and in need of life-saving treatment having become "full". Hours later when normal service had been restored and the full post-mortem began we discovered what had happened.
At the time we had no remote access into email so one Thursday a contractor, prior to going home for a three day weekend, set up an auto-forward rule in his mailbox to forward any incoming mail to his Hotmail account and an account with another ssupplier (can't remember who). This despite a total ban on auto-forwards to outside the company.
Everything was fine throughout Friday; emails arrived and were forwarded to his two private accounts. Unfortunately at some point on Saturday morning his Hotmail inbox became full and so the next time it received a message from his work account it replied saying that it was undeliverable because the mailbox was full.
So our system duly forwarded that to his Hotmail account...
Which replied that it was full...
Repeat this loop until some time later his other account became full...
...and started sending replies that it couldn't accept incoming messages because it was full. Which our server dutifully forwarded to BOTH accounts which BOTH replied that they were full...
And some time on Sunday our mail servers gave up because THEY were full.
Oh how we laughed! Or maybe we didn't. The contractor had his contract terminated that day.