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. IOFM estimates that errors push the cost per invoice processed up by as much as 20%.

According to a Gartner study, 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 the 8 errors we see most often 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.

Sergio, accounting manager at a transport company in Murcia, discovered during a tax audit that 14 invoices from one supplier had a tax ID with one digit changed. The supplier had updated its system and the tax ID was generated with a typo. Nobody verified it for 5 months. The cost: VAT regularisation and EUR 1,800 in 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.

According to IOFM, automatic detection prevents up to 95% of duplicates. Without it, companies pay between 6 and 12 duplicate invoices per month, with an average of EUR 1,900 per duplicate.

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.

How to avoid it: automatic cross-check of four fields (invoice number, supplier, amount, date) on every incoming invoice. Three out of four match, alert. Four out of four, block.

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.

An Ardent Partners study (restricted-access report) puts the share of invoices still processed manually at 49.7%. APQC 2026 benchmarking confirms that 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.

Three-way 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 three general rates (21%, 10%, 4%) 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.

Carmen, finance director at a consulting firm in Madrid, found that during a full quarter 21% VAT had been applied to invoices from a Canary Islands supplier exempt from IGIC. The non-deductible VAT cost: EUR 3,400.

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, the system distributes them automatically. If an invoice sits unprocessed for more than 5 days, an alert is raised.

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

In Spain, Law 15/2010 sets a 60-day maximum payment term for suppliers. Companies that miss it appear on public late-payer lists and face penalties. Since 2022, the government regularly publishes companies that exceed the limits.

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.

Diego, CFO of a packaging manufacturer in Alicante, found that 30% of his invoices were paid after day 60. It was not a treasury problem; it was a process problem. Invoices took 22 days on average to flow through internal approval. When he automated the flow, the time dropped to 3 days.

How to avoid it: automated approval flow with defined deadlines. If an approver does not respond in time, the invoice escalates to the next level. The system does not let an invoice stay “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 automatic escalation when the deadline is missed.

How to avoid it: define the flow in writing and, if possible, implement it in a system that runs it automatically. 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

Maria runs invoices. Maria knows where every document is, which suppliers are problematic, which POs are pending and which invoices to chase. When Maria is on holiday, everything stops.

This is not Maria’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 Maria leaves the company, the knowledge leaves with her. 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.

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. Not a theoretical number. It is what companies without automatic controls in their invoice process actually pay.

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 automatic escalation.

ininvoice does all three: centralises intake from your email, cross-checks every invoice line by line with POs and delivery notes, detects duplicates by four fields and applies configurable tolerances. 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 how 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.