During my days as a Computer Engineer, fortunately I was not the culprit, but I saw a few things that were pretty bad but nowhere near this bad.
On the Engineering team I was on, we had a person take down the entire ATM network along the eastern coast of the USA, several thousand ATMs in total, this was one of the largest bank in America, fortunately we recovered from that mistake in about 1 hour.
We had a guy one time take down about 20,000 voice mail accounts at the corporate office including the CEO of the same bank where the ATMs were taken down.
Finally, we had a guy while working at the same bank, while running a cleanup routine on Active Directory accounts accidently deleted thousands of user accounts of active users, rendering their access to the network null and void, that was a major screw up.
Wow, thats pretty big oops.