The 8 most common mistakes when managing supplier invoices (and how to avoid them)

39% of invoices contain some kind of error. Not necessarily severe errors, but discrepancies that someone has to detect, investigate and correct. And when nobody does, they pile up. Errors are more frequent in sectors with high purchasing volume.

Think about your finance department. Every mishandled invoice is not just a wrong number in a spreadsheet. It is overpayment, a missed discount, a damaged supplier relationship, or a penalty for late payment. Errors push the cost per invoice processed up by as much as 20%.

18% of accounting professionals admit to making mistakes daily. And 33% recognise several errors per week. It is not incompetence; it is volume, urgency and manual processes that do not scale.

These are 8 of the most common errors at SMEs that manage supplier invoices, why they happen, and how to prevent them before they cost money.

Error 1: not verifying basic invoice data

The simplest and most frequent error. The invoice arrives, gets posted and is approved without anyone checking that the basic data is correct: supplier tax ID, invoice number, issue date, line descriptions, unit prices.

A wrong tax ID invalidates the invoice for tax purposes. A wrong issue date may affect the VAT period in which it is declared. A duplicate or badly formatted invoice number breaks any traceability system.

Constructed example: if a supplier changes its invoicing system and its tax ID comes out with one digit changed, a company that does not check it on entry could pile up months of invoices with that error. If a tax audit finds it, the company would have to regularise the VAT on all of them and could face surcharges.

How to avoid it: automatic verification of the tax ID against the tax authority database when posting the invoice. If the tax ID does not match a registered supplier, the invoice is held.

Error 2: posting duplicate invoices

The same invoice enters the system twice. Once by email, once by post. Or the supplier resends because they got no confirmation. The result: double payment.

Without a control that compares each new invoice with those already recorded before payment, the duplicate only surfaces when the supplier says so or when the bank is reconciled.

This is not a minor issue. If you want to understand the causes in detail and how to detect them, we have a full guide on duplicate invoices, and the duplicate invoice controls that catch them at intake rather than after payment.

How to avoid it: automatic cross-check of four fields (invoice number, supplier, amount, date) on every incoming invoice. If the number from the same supplier matches, or the amount and the date match, the later one is held and a person decides. If only the amount matches on nearby dates, it warns without holding.

Error 3: not cross-checking the invoice with the PO and the delivery note

This is the error that costs the most money over time. The invoice is approved without checking that it matches what was ordered (purchase order) and what was received (delivery note). You pay what the supplier says, not what was agreed.

The share of invoices still processed manually stands at 49.7%. Bottom-quartile organisations spend over 10 days processing an invoice, vs 3.7 days in the top quartile. Without systematic matching, price and quantity discrepancies go unnoticed.

invoice-to-PO-to-receipt matching exists precisely for this: cross-check every invoice line with the PO line and the delivery note line. Without this check, you are trusting that the supplier always invoices correctly. And the data says they do not, 39% of the time.

How to avoid it: line-by-line reconciliation, not totals only. If line 3 of the invoice says EUR 0.09/unit but the PO said EUR 0.08, that discrepancy has to surface before payment.

Error 4: applying VAT or withholdings incorrectly

VAT in Spain has one general rate (21%) and two reduced rates (10% and 4%) (Law 37/1992, arts. 90 and 91), plus exemptions and special regimes. Personal income tax withholdings affect freelance invoices. Mistakes are easy, especially with suppliers under different regimes or with intra-EU operations.

A wrong VAT does not only affect the amount you pay; it affects your quarterly return. If you declare input VAT that does not apply, the tax authority will catch it.

Constructed example: a consulting firm receiving invoices from a Canary Islands supplier with 21% VAT that does not apply to the transaction could record them that way for a full quarter if nobody checks the supplier’s tax regime. That wrongly charged VAT would not be deductible, and the supplier would have to issue a corrective invoice.

How to avoid it: tax validation rules per supplier. When a supplier is set up, its tax regime is recorded. If the invoice arrives with a non-matching tax rate, an alert is raised.

Error 5: losing invoices between departments

The invoice lands in purchasing. Purchasing forwards it to administration. Administration passes it to accounting. Somewhere along the line, it is lost. Or it sits in the inbox of someone on holiday. Or it is filed in the wrong folder.

The result: the supplier chases, the team searches, nobody finds the original invoice, and someone has to ask for a copy. Time wasted, relationship damaged, and sometimes a late payment.

61% of late payments are caused by internal mistakes, not lack of funds. The invoice existed; simply nobody processed it in time.

How to avoid it: single intake point. All invoices arrive through the same channel (a dedicated email, a portal). From there, nothing depends on someone forwarding an e-mail, and any invoice sitting unprocessed for more than 5 days should be chased.

Error 6: paying late (and assuming it is normal)

In Spain, Law 3/2004 on late payment (art. 4) sets the payment term for suppliers at 30 days from receipt of the goods or services, extendable by agreement to a maximum of 60. Paying later accrues statutory late-payment interest and compensation for recovery costs under the same law.

But many companies pay late not because of liquidity, but because their approval process is slow. The invoice arrives, queues, goes through three approvers who each take a week, and when it is paid, 75 days have passed.

Constructed example: in a manufacturer where each invoice waits weeks in the internal approval flow, part of the invoices would be paid after day 60 even with cash available. It would not be a treasury problem but a process problem.

How to avoid it: an approval flow with defined deadlines and a named deputy for each approver. If an approver does not respond in time, the deputy steps in, so no invoice stays “in limbo” indefinitely.

Error 7: not having a clear approval flow

Who approves what? Above what amount do you need a second signature? What happens if the approver is on holiday?

At many companies, the answer is “it depends.” It depends on who is available, how urgent it is, whether someone remembers. That ambiguity creates delays, unreviewed approvals and, in the worst case, unauthorised payments.

A clear approval flow defines: amounts and authorisation levels, deputies for each approver, maximum approval/rejection time, and what happens when the deadline is missed.

How to avoid it: define the flow in writing and, if possible, use a system that records every approval. It does not have to be complex: a rule like “invoices above EUR 5,000 require finance director approval” is already a flow.

Error 8: depending on one person for the whole process

A single person runs invoices. They know where every document is, which suppliers are problematic, which POs are pending and which invoices to chase. When they are on holiday, everything stops.

This is not that person’s fault. It is an organisational error. If the invoice process depends on one person’s tacit knowledge, the process is not a process; it is a person.

The risk goes beyond holidays. If that person leaves the company, the knowledge leaves with them. And rebuilding the process from scratch, undocumented, can take months.

We have a dedicated article on this issue in the context of companies still relying on Excel for invoice management.

How to avoid it: document the process, standardise the reconciliation rules and use a system where information is available to the whole team, not in one person’s head.

The accumulated cost of these errors

None of these errors sinks a company on its own. But together, the cost adds up fast.

Estimate with assumed frequencies for a company with 400 invoices per month:

Error	Estimated frequency	Cost per incident	Monthly cost
Duplicate invoice paid	4-8/month	EUR 1,200	EUR 4,800-9,600
Undetected price discrepancy	15-20/month	EUR 50-200	EUR 750-4,000
Late payment (surcharge)	5-10/month	EUR 30-100	EUR 150-1,000
VAT misapplied (regularisation)	2-3/quarter	EUR 500-2,000	EUR 300-600
Lost invoice (search time)	5-10/month	EUR 50 (time)	EUR 250-500
Total estimate			EUR 6,250-15,700/month

Per year: between EUR 75,000 and EUR 188,000 in avoidable costs, under these assumptions. It is an estimate, not a measured result: replace the frequencies with your company’s to get your figure.

How to prevent the 8 errors at once

The 8 errors share a common denominator: manual processes, scattered information and lack of automatic verification. Prevention does not require 8 different solutions. It requires three things:

1. Centralisation. All invoices enter through a single point. From there, they are processed in a standard way. No exceptions, no shortcuts.

2. Automatic verification. Every invoice is matched against the PO and the delivery note (three-way matching), checked against previous invoices (duplicate detection) and validated fiscally (tax ID, tax rate).

3. Defined approval flow. Clear rules on who approves what, with deadlines and deputies.

ininvoice covers centralisation and verification: it centralises intake from your email, cross-checks every invoice line by line with POs and delivery notes, detects duplicates by four fields and applies the tolerance set by your organisation. Approval is made by a person, and approved invoices are exported as CSV or Excel for your ERP. It is not the only option in the market. But if you process more than 200 invoices a month and you recognise three or more of these errors, you need something beyond Excel and manual review.

FAQ

What is the acceptable error rate on invoices?
The industry standard puts the acceptable rate below 5%. Best-in-class companies achieve less than 1%. If you are above 3%, you have a process problem, not a people problem.

Can all errors be prevented with software?
No. Software prevents systematic errors: duplicates, price discrepancies, invoices without POs. But judgement errors (approving an invoice that should not be approved) require training and human oversight. Before buying, review what to look for when comparing AP software.

How long does it take a company to notice it has an error problem?
Usually, until an audit or a supplier complaint. The companies that actively measure their error rate are the ones with the fewest errors, precisely because they catch them early.

Is an approval flow mandatory?
No law requires it. But any auditor, internal or external, expects it. And internal control frameworks (COSO, SOX) treat it as a baseline control.

Conclusion

Supplier invoice errors are not inevitable. They are the result of manual processes, scattered information and missing automatic controls. The 8 errors in this article repeat themselves at most SMEs that process invoices without automation.

The fix is not complicated. Centralising intake, cross-checking documents automatically and defining a clear approval flow eliminates most errors before they turn into costs.

If you recognise three or more of these errors in your company, the problem is not your team. It is your process. And the process can change. Check the way ininvoice works and review the available features to start reducing them.

Related reading: how to handle supplier invoice exceptions, how to calculate price variance between invoice and PO, and the difference between received invoice and PO when amounts differ.