Most engineers and analysts treat a mortgage calculator as a black box: enter three numbers, get a monthly payment back, walk away. When something looks wrong on a loan estimate, the instinct is to suspect the bank, the broker, or the tool itself — without a way to verify which one. This article takes the opposite approach. It treats the calculator's full amortization schedule as a dataset you can audit, and lays out a practical checklist for spotting inconsistencies, reconciling the output agai...