An example
=C2*D2 in every row, except row 14, which holds 1,250
Row 14 was overwritten with the figure it showed that day. When its inputs change, it will not.
How it happens
A quick fix: the formula gave an answer somebody disagreed with, so they typed the one they wanted. Or values were pasted over part of a column.
Why it matters
The cell looks exactly like its neighbours. It was right on the day it was typed and becomes more wrong each time the figures it should depend on change.
How to find it by hand
In Excel, Find and Select, then Go To Special, then Constants with only Numbers ticked, selects every typed number on the sheet. Look for any that sit inside a column of formulas.
In Google Sheets, show formulas with Ctrl and the backtick key and read down each calculated column for a plain number among the formulas.
How Proper Spreadsheets finds it
The checker reports any typed number sitting inside a run of the same formula, with the formula the rows around it use, so you can put it back.
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.
- One row that calculates something different: Every row in the column does the same thing, except one. The answer it shows looks perfectly normal.
- 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.