In my experience this happens frequently enough that there ought to be an article like this for every problem domain that software developers and executives und

In my experience this happens frequently enough that there ought to be an article like this for every problem domain that software developers and executives under-estimate the complexity of. I work on card transaction processing and it's absolutely more complicated than you can imagine looking at it from the outside. It's a vast, distributed peer-to-peer system where trust is built into liabilities outside of the network itself. You want to build a system that is correct with regards to some specification in order to protect peoples' money... but the problem is that there is no specification that enumerates every possible transaction state because while there are "typical" sequences of messages to handle, your system also has to handle unexpected sequences that can't be specified: there are no guarantees that systems on the other end are emitting properly formatted events let alone behaving as expected. Card payment handling systems have to have the ability to manually correct and adjust state by humans; reconcile with external systems (because yay, payment systems in the US aren't based on instantaneous settlement until the roll-out of FedNow is complete), etc. I've watched many businesses walk straight into, it's just X, how hard can it be? Only to watch their ARR shrink and stress levels rise when a project they thought would take a month turns into a year-long journey of discovery, reflection, and increased head-count.