Something wrong with some numbers in that spreadsheet? Here's a shortcut that displays all the formulas in the sheet at once:
Ctrl + ' (the acute accent key, found next to the number 1 on your keyboard). On a Mac, try Cmd + '
That's it! With the formulas displayed, you can click around to see which cells and functions are being referenced. You can toggle back to the values view by hitting the shortcut again.
Troubleshooting misbehaving numbers? [Twitter @Office]