The balance column stopped matching. Here is the row where it broke.

Drop a register — a check book, a petty cash sheet, any export with an amount column and a running balance beside it. This walks the two columns together and stops at every row where the balance does not move by the amount next to it, then tells you what the difference looks like: a sign flipped, an amount applied twice, or two digits swapped.

Drop a register CSV here, or click to choose one Needs an amount column and a balance column. Nothing is uploaded.

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What it checks

One thing, precisely: for every row, does previous balance + this amount equal this balance? That comparison localises the problem to a single line, which is the part that takes a human an hour of squinting.

It deliberately measures each step against the stated previous balance rather than a running total of its own. Those differ in an important way: a total of its own would mark every row after the first mistake as wrong, burying the one row you need. Measuring step by step means three separate typos show up as three findings instead of one finding and two hundred false ones.

What the difference tells you

The size of the gap is usually a fingerprint, so each break is labelled where the arithmetic supports it:

A sign was flipped. The gap is exactly twice the amount. A payment was added where it should have been subtracted, or the reverse — much the most common cause, and invisible if you are only scanning for a wrong-looking number.
The amount was applied twice, or not at all. The gap equals the amount exactly, in one direction or the other.
Two digits were transposed. The gap is divisible by nine and the two balances contain the same digits in a different order. The divisibility rule on its own flags every honest $9.00 fee, so both tests have to pass.
A row is missing. Nothing above fits and the balance moved anyway — usually a transaction that was never entered.

What it does not do

It reads and reports; nothing is written and there is nothing to save. It will not repair the column for you, because the correct fix depends on which of the two numbers is the true one — and only you know whether the bank agrees with the amount or with the balance.