Numbers stored as text, and the total that is quietly short.

An error is loud. This is not: the sum returns a number, the number is plausible, and nothing warns you. A total that is four per cent short passes every sanity check a person applies by eye.

It is also self-concealing over time. Once a shortfall is reported once, it becomes the baseline, and the following month's comparison is between two figures that are both wrong by different amounts.

A currency symbol inside the cell rather than applied as a format. A thousands separator that came from a system using a different one from yours. A trailing space, or worse a non-breaking space, which is invisible and common in anything copied from a web page.

Accounting-style negatives wrapped in parentheses are the fourth, and they are the most consequential because those rows are usually the refunds and the credits — so the total is not merely short, it is short in one direction.

That directionality is what makes this worth catching rather than tolerating. A random shortfall averages out across a year and a systematic one does not: if every negative adjustment is being skipped, revenue is overstated in every period, by a growing amount as the business grows.

Count how many cells in the column are numeric and compare that against how many are non-empty. If the two disagree, the difference is the size of your problem.

Converting them is straightforward. Preventing them from arriving again is the part that matters, and it is almost always an export setting in the system that produced the file rather than anything about the spreadsheet.

Where the file comes from somebody else's system and cannot be changed, record the conversion as part of the process rather than doing it by hand each month, because doing it by hand is how one month gets skipped.