Fire alarm on a client-facing budget summed to $19,600 across its line items. The printed division total was $18,620, exactly five percent lower. Fire sprinkler carried the same cut. Specialties was off by about 21 percent. Equipment was off too.
The cuts had been applied to division totals during a value-engineering pass, and the line items underneath were left alone. Anyone who checked the arithmetic could see it. The language model that helped produce the VE list also left broken #REF! formulas in the workbook.
Parse the workbook in code, run the extensions and roll-ups in code, and report every check with the cell it came from. The model is there to read a layout that is not standard.
The same desk caught a cost-per-square-foot figure on a competitor's budget that would not divide evenly into any area on the sheet. The number came from an older, stale area. Once the workbook is parsed, both misses are a sub-second check.
On a later review call, that firm's model returned a roofing cost that was simply wrong. The remedy they adopted is a company cost book: every unit price needs a source, and an update supersedes the old row while the history is kept. The assistant can propose a change to a price. A person approves it before it becomes company data.
A model working from general training data will produce a confident roofing number on request. It will not produce last quarter's buy-out.