[yellowstuff]

I don't fully understand what happened, but it sounds like essentially someone was supposed to update a payment from "repay full amount" to "pay interest only", and didn't do it. The system correctly executed the instructions it was given. The only way this would show up as a "loss" to the bank is if some part of the system expected to pay only the interest, and it sounds like that wasn't the case.

It also sounds like there was another human who was supposed to catch the error and didn't. My guess is that the volume of transactions is high enough and errors of this type are rare enough that most people wouldn't be able to catch it manually.