10 common bookkeeping mistakes small businesses make, and how to fix each one

Why the same mistakes keep coming back

We take over books from founders, from part-time bookkeepers and from software that was left to run by itself. The problems are almost always the same ten. None of them needs an accounting degree to fix. They come from doing the books in a hurry, once a quarter, with nobody looking at the result. That is why the fix for most of them is the same: close the books every month, and have a second person check them.

The ten mistakes, and the fix for each

1. Personal and business money in one account. Every month someone has to work out which lunch was a client meeting, and tax time becomes an argument with yourself. Fix: one business account and one business card from the first day. Pay yourself by transfer, not by running personal costs through the company.
2. Skipping the monthly bank reconciliation. If the bank balance in your software does not match the statement, nothing built on it can be trusted. Fix: reconcile every bank, card and payment processor account to its statement every month, and leave any difference open until it is explained. The month-end close process shows where this sits.
3. Accepting the bank feed's categories without a look. The software guesses what each line is, and the guess is often wrong: a loan repayment booked as an expense, a customer refund booked as income. Fix: a person reviews every line before it is posted. The feed saves typing, not judgment.
4. Reading profit as cash. A profitable month can still end with an empty account, because customers pay late and payroll, suppliers and tax do not wait. Fix: look at the bank balance and the open invoices together, and keep a weekly cash forecast once payroll is real money.
5. Booking customer prepayments as revenue. A customer pays a year upfront and the month looks wonderful. The next eleven look poor, and the balance sheet forgets that you still owe the work. Fix: record revenue as it is earned, and keep the unearned part as a liability. Cash basis versus accrual explains the difference.
6. Losing the paperwork. A bank line with no invoice behind it is a number with no proof. Auditors, lenders and tax authorities ask for the document, not the entry. Fix: one shared folder or receipt tool, and the rule that nothing is posted without its document attached.
7. Paying people as contractors who work like employees. The label on the contract does not decide this. The IRS looks at the whole relationship: who controls how the work is done, who carries the business risk, and whether the work is ongoing and central to the company. Getting it wrong means back taxes and penalties. Fix: review every long-term contractor once a year with your tax adviser.
8. Ignoring sales tax until a state writes to you. Since 2018, most US states can require an out-of-state seller to collect their sales tax once its sales into the state pass a threshold. Online sellers cross those lines without noticing. Fix: track sales by state from the first online order, and register before the letter arrives, not after.
9. A growing balance in suspense or "uncategorized". Every line in there is a decision somebody postponed. Left alone, it becomes hundreds of lines nobody can remember. Fix: empty these accounts as part of every close. If the same charge lands there each month, fix the rule that sends it there.
10. Nobody reads the numbers, and nobody checks the person who does them. Reports that go into a drawer catch nothing. One person doing everything with no review is how errors, and sometimes fraud, live for years. Fix: a short monthly review of the profit and loss, balance sheet and cash, and a second person who signs the close.

How to tell which ones you have

You do not need to audit yourself. Each mistake leaves a mark in the numbers. Look for these.

Where AI helps, and where it does not

In our own work AI does most of the matching, sorting and first-draft categorizing. It is quick, it does not get tired, and it flags a line that looks unlike the ones before it. That takes care of the volume behind mistakes 2, 3 and 9.

It does not fix mistakes 1, 7, 8 or 10, because those are decisions and habits. And it makes its own kind of mistake, quietly. One of our own AI-run bank imports once dropped seven rows without any error message. A person caught it, because a person compared the import with the source file. That check is now a written rule for us.

Fix them in this order

Start with the bank reconciliation. Once cash is right, every other problem becomes smaller and easier to see. Then clear suspense, then fix revenue timing, then the tax questions. If the books are more than a few months behind, clean them up in this order first, or let us do it and hand you a set of books where every balance ties.

Where this goes wrong

First published 2025. Rewritten and checked in September 2026. If something here is out of date, tell us and we will fix it.