The biggest face palm-worthy exercise I can recall participating in was a roll-in of Oracle Mail years ago, replacing 2 different mail systems that I can't even recall now (this was pre-Y2k ... well before).
One enterprising young DOE tech (no, was NOT me) put in place a test scenario to prove out the store and forward feature, ensuring that no emails would be lost in the transition off the old systems. However, in the very first test to a small list, he somehow activated an AUTO REPLY ALL to the whole enterprise, which ended up in an endless loop between all 3 systems and blew them all up.
:double facepalm: