An example
Row 7: =C7*D7, row 8: =C8*D7, row 9: =C9*D9
Row 8 multiplies by the rate in row 7 instead of its own. Its answer is close enough to pass a glance.
How it happens
A cell edited to fix one case and never put back. A paste that shifted a reference by a row. A formula copied in from another file. Each leaves one row doing something its neighbours do not.
Why it matters
Nothing shows an error, and the figure is plausible. It is the kind of mistake that survives every review that looks at results rather than formulas.
How to find it by hand
Excel marks some inconsistent formulas with a small green triangle in the corner of the cell, but only for some patterns and only when error checking is switched on. Google Sheets has no such marker.
The dependable way is to show formulas with Ctrl and the backtick key and read down every calculated column, row by row, looking for the one that differs.
How Proper Spreadsheets finds it
The checker rewrites every formula in a form where a filled down column has a single shape, and reports any row inside a run whose shape differs, saying what the rows around it do. The first and last rows of a run are left alone, because an opening balance or a total legitimately differs.
Check a whole file in seconds
Finding this by hand means reading every formula in the file. The free check reads them all, along with six other mistakes that show no error, and points to each cell. Your file is deleted as soon as the check finishes.
Other mistakes worth checking for
- A total that stops short of the data: The formula is fine. It just ends a few rows too early, and nothing on the screen says so.
- A number typed into a formula: It is right until the rate changes. Then it is wrong in every row it was copied into.
- A figure typed over a formula: The column calculates, row after row, until the one row where somebody typed the answer in.
- A total that disagrees with its rows: The rows add up to one figure and the total says another.
- Error values, and the one cell causing them: Two hundred cells show #REF!. One of them is the cause, and it is rarely the first one you see.
- A formula that points at a sheet that is gone: The cell shows 0. It should show an error, and would, if IFERROR were not hiding it.