If you've got 99 cells with identical formulas, and one without, it's not far fetched for Excel to speculate that there may be a mistake (particularly if they're within X% of being identical). At which point it can provide a simple, non-intrusive hint.
And going further up the wishful thinking tree, into more difficult territory, Excel should be able to grasp what you're attempting to do by understanding, say, the top 10,000 (arbitrary number) tasks commonly performed by people using spreadsheets. That 'understanding' could then tip Excel toward deciding whether there's a problem, and whether it should leave you a nice little illuminated icon at the top right of the program with an exclamation mark.
Microsoft should have insane amounts of data on everything people do with spreadsheets, and they should be able to make it radically more intelligent (without being obnoxious or intrusive - it should be optional and highly passive).