The invoice mistakes that cost small businesses the most are simple ones: the wrong client name, a missing due date, a reused invoice number, an out of date tax rate, a discount taken after tax and editing an invoice that was already sent. Each one delays payment or causes tax trouble, and each has a quick fix you can build into your routine.
The 12 invoice mistakes at a glance
Here is the full list with the fix for each. The sections below explain the ones that need more than a line, with examples. Most of these take seconds to catch with a short check before you press send, which is the last section of this guide.
| # | Mistake | Fix |
|---|---|---|
| 1 | Billing the wrong person or company | Confirm the legal name and billing contact at the quote |
| 2 | Leaving out the client’s PO number | Ask for it up front and print it near the top |
| 3 | Vague line descriptions | Say what, when, how much and how many |
| 4 | Reused or skipped invoice numbers | One unbroken sequence, never edited by hand |
| 5 | No due date, or only “net 30” | Print the actual date next to the total |
| 6 | Missing or wrong dates | Show issue date and supply date when they differ |
| 7 | Calling it a tax invoice when you are not registered | Use the word “Invoice” and charge no tax |
| 8 | Missing your tax number | Add it to your saved business details |
| 9 | Using an old tax rate | Check rates on the tax authority site each year |
| 10 | Taking a fixed discount after tax | Discount first, then work out tax |
| 11 | Rounding tax inconsistently | Pick per line or on the subtotal, and stick to it |
| 12 | Editing an invoice already sent | Issue a credit note and a new invoice |
Mistakes 1 to 3: client details and descriptions
An invoice addressed to the wrong entity or missing the client’s purchase order number often sits unpaid in an accounts inbox until someone asks about it. Vague descriptions cause the same pause. Get the billing details right at the quote stage and describe the work so a stranger could understand it.
- 1. Wrong person or company. A director may hire you, but the invoice may need to go to their company, by its legal name, to a separate accounts email. Ask “Who should the invoice be addressed to, and where should I send it?” before you start.
- 2. Missing PO number. Some larger clients cannot pay an invoice without one. Ask for it when the job is booked.
- 3. Vague descriptions. “Services, $1,500” invites questions. “Bathroom tiling, 14 sq yd, labor and grout, Nov 9 to 11: $1,500.00” does not.
Our checklist of what to include on an invoice lists every field, and how to write an invoice walks through filling them in.
Mistakes 4 to 6: numbers and dates
Numbering and dates are where tax rules are strict and records go wrong. Every invoice needs a unique number in a sequence you can explain, and dates that show when you issued it and when payment is due. Where the supply date differs from the issue date, some countries want both.
- 4. Reused or skipped numbers. Sending two invoices as INV-0102 breaks your records and confuses the client’s accounts team. The UK’s VAT guide asks for a sequential number based on one or more series that uniquely identifies the document. Keep one series and note why if a number is ever voided.
- 5. No due date. “Net 30” leaves the client to work out the date. Write “Due December 4, 2026” instead. Our due date guide explains the options.
- 6. Missing or wrong dates. The UK list includes the time of supply and the date of issue where they differ. Ireland asks for the date the goods or services were supplied.
The invoice numbering guide covers formats such as INV-2026-0102 and how to restart numbering safely.
Mistakes 7 to 9: tax wording, tax numbers and rates
Tax errors are the costliest kind, because they can affect your client’s tax claim as well as your own return. Use tax wording only when you are registered, show your tax number when the rules ask for it, and check the rate each time a government announces a change.
- 7. “Tax invoice” when you are not registered. In Australia, business.gov.au says a business not registered for GST must not use “tax invoice” on its regular invoices. Title it “Invoice” and leave tax off.
- 8. Missing tax number. Required details lists include it: the ABN in Australia, the VAT registration number in the UK, the GST registration number in Singapore. Save it once in your business details so it appears every time.
- 9. Old tax rates. Rates do change. Nova Scotia’s HST fell from 15% to 14% on April 1, 2025, and in Ireland hairdressing and some catering moved from 13.5% to 9% VAT from July 1, 2026. A template made before those dates would charge the wrong rate.
Our page of invoice requirements for ten countries lists current rates and the required details, with sources.
Mistakes 10 and 11: discounts and rounding
A fixed amount discount must come off before tax is worked out, or the tax shown is wrong even if the total looks close. Rounding needs a consistent rule: either round tax on each line or on the subtotal. Mixing the two produces totals that are a cent or two out.
10. Discount after tax
| Step | Correct | Wrong |
|---|---|---|
| Price | $1,000.00 | $1,000.00 |
| Discount of $100 | $900.00 | applied later |
| Tax at 20% (example rate) | $180.00 | $200.00 |
| Total | $1,080.00 | $1,100.00 after the $100 discount |
The wrong version overcharges by $20.00 and reports $20.00 too much tax.
11. Inconsistent rounding
Three items at $10.45 with 15% tax: tax per line is $1.57 each, so $4.71; tax on the $31.35 subtotal is $4.70. Singapore’s IRAS allows either method, used consistently. Our invoice generator does the arithmetic for you, so the totals match every time.
Mistake 12 and other sending errors
Once an invoice is sent, treat it as fixed. If something is wrong, issue a credit note that cancels all or part of it, then send a corrected invoice with a new number. Editing the original quietly leaves you and the client holding different versions of the same document.
- 12. Editing a sent invoice. Use a credit note for the difference. Our guide to credit notes explains how, and the credit note generator makes one.
- Sending late. An invoice sent weeks after the job starts the payment clock weeks late, and some tax rules set a time limit for issuing it.
- Sending a Word file. Editable files can shift on the client’s screen. Send a PDF.
- Foreign currency without conversion. In the UK, the total VAT must be shown in sterling even if the lines are in another currency.
A 60 second check before you send
Run the same short check on every invoice before it leaves. It covers every mistake on this list, since each one shows up on the page if you look for it. Read the invoice as the client’s accounts person would, from the top down.
- Client legal name, address and billing email are right
- PO or job reference included if the client uses one
- Invoice number is the next in sequence and not already used
- Issue date, supply date if different, and a real due date
- Each line says what, when and how many
- Your tax number shows if you are registered
- Tax rate is current and discounts come before tax
- Subtotal, tax and total add up
- Payment details and reference are complete
- Saved and sent as a PDF
For more on avoiding slow payments, see how to get paid faster, or browse all our invoicing guides.
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