Comment by PeterStuer

This was many years ago. I has an injurie that prevented me from driving, so I asked if there was any project on the backburner I could do on my own in 3 weeks without too much interaction (this was long before easy remote work infra).

Sure there was. A financial services company wanted to replace their repayment plan generator that ran on an aging AS/400 with someting running in .Net on Windows.

I dug in an learned all about time value of money, numerical formats and precision, rounding strategies, day counting strategies for incomplete periods (did you know there are many dozens of those) etc.

I made everyting as configurable as I could so we had the option to offer to other financial service clients by just setting up a different profile.

Since I had no interaction with the client before the first presentation meeting, I put in what to me felt like the most plausible config (I had managed projects in the domain before so I was not completely clueless to the mindset).

We had the meeting. I showed a few calculated plans which they compared on the spot to the AS/400 output. I deployed on a test VM for them so they could do more extensive testing. Code was accepted with 0 change requests and put into production shortly thereafter. Don't think they ever changed from the default settings.