I'm surprised (or I'd be surprised) that there isn't a way to automatically audit Excel sheets for these sorts of mistakes. I know Excel will highlight if a formula isn't consistent with the other formulas in the column or if a cell is formatted as text and others aren't. There has to be (or should be) a tool to centralize that check.

Another problem I could see in Excel (and reports in general): using a hard-coded value. E.g. 31 because that's how many sales areas there are. Works great. When 31 becomes 30, the report is close enough no-one notices. How long, and how wrong, does the report get before anyone notices?