I mean...if this were any other business, someone clicking the wrong button and then someone else not catching this error would maybe mean someone's account was accidentally closed, or someone receiving a free pizza, or whatever.

The IT systems within banks are more or less the same as IT systems anywhere. Just as advanced, just as crappy. The difference is that if there is a human error with banking software, you're not sending free pizza, you accidentally pay ouy $900,000,000.