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 spreadsheet reference 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.