KSeF 2026 – the 5 most common mistakes in structured invoicing and how to avoid them

The mandatory National e-Invoicing System (KSeF) fundamentally changes the way transactions between businesses are documented. In practice, this means not only new rules for issuing invoices, but also the need to adapt accounting processes, internal procedures and IT systems.

While most businesses focus primarily on the technical implementation of KSeF, experience from the first months of the system’s operation demonstrates that the greatest risks do not arise from submitting documents to the system itself, but rather from the incorrect treatment of non-standard scenarios.

Errors in VAT identification numbers, incorrectly issued settlement invoices, incomplete collective corrective invoices or discrepancies between the XML file and the PDF visualisation may result not only in operational issues but also in disputes with the tax authorities.

Below we outline five key areas that deserve particular attention.

1. Incorrect VAT identification number on an invoice – not every error requires the same type of correction

One of the most common errors is the inclusion of an incorrect VAT identification number (NIP). In practice, many taxpayers assume that every such error must be corrected in the same manner. However, both the legislation and the current position of the tax authorities indicate that the appropriate course of action depends on the nature of the error.

Where the incorrect VAT identification number relates to the actual purchaser, the tax authorities take the view that a corrective invoice reducing the original invoice to nil should first be issued, followed by the issue of a new invoice documenting the transaction with the correct purchaser details.

The position differs where the incorrect VAT identification number relates only to an organisational unit or another additional entity identified on the invoice. In such cases, it is generally possible to correct only the incorrect identification details without cancelling the entire invoice.

This distinction is of considerable practical importance, as it affects both the method of documenting the transaction and the timing of the VAT settlement.

Practical tip

Before issuing a corrective invoice, it is advisable to determine whether the error concerns the actual purchaser or merely supplementary descriptive information. This helps avoid the unnecessary issuance of additional documents.

2. Settlement invoices – the most common errors

Settlement invoices issued following the receipt of advance payments require particular care.

Within the KSeF environment, it is no longer sufficient for a document merely to comply with the provisions of the VAT Act. It must also satisfy the requirements of the FA(3) logical structure.

The most common errors include:

• showing the full value of the sale instead of only the outstanding amount payable;
• incorrect VAT settlement following earlier advance payments;
• failure to link the settlement invoice with the relevant advance payment invoices; and
• incorrect designation of the invoice type.

It should be remembered that the final invoice, in the section summarising values by VAT rate or transaction type, as well as the total amount payable, should reflect only the outstanding balance remaining after the advance payments have been settled. It should not repeat the full value of the transaction. However, at the invoice line level, which contains the detailed breakdown of the goods and services supplied, the full value of the transaction items should be presented.

Failure to prepare the document correctly may result in its rejection by KSeF before a system reference number is assigned.

3. Employee purchases – why an internal procedure is essential

In many businesses, employees make day-to-day purchases on behalf of the company. These commonly include:

• fuel;
• consumable materials;
• minor construction supplies;
• catering services; and
• office supplies.

Following the implementation of KSeF, all such invoices are delivered directly to the company’s account within the system.

The difficulty is that the invoice itself does not always identify which employee made the purchase.

Without appropriate procedures, businesses may encounter:

• delays in settling business travel expenses;
• difficulties in allocating costs;
• issues with expenditure approval; and
• the risk of losing supporting documentation.

How can the risk be mitigated?

A practical solution is to require employees to provide the supplier with additional information identifying the purchase.

Such information may include:

• employee number;
• cost centre (MPK);
• vehicle registration number;
• department name; or
• project number.

At the same time, the business should maintain records enabling each invoice downloaded from KSeF to be linked to the relevant employee.

4. Collective corrective invoices – simplifications do not remove compliance obligations

In practice, many businesses use collective corrective invoices to document periodic rebates.

A common misconception is that it is sufficient simply to indicate the period to which the rebate relates.

This approach goes too far.

Although the legislation permits certain simplifications, it remains necessary to identify clearly every invoice covered by the correction.

In other words, full traceability of the relevant documents must be maintained.

Before implementing KSeF, businesses should verify whether their accounting and finance system correctly generates collective corrective invoices in accordance with the current requirements.

5. The XML file and PDF representation must be consistent

One of the less obvious risks is a discrepancy between the data contained in the XML file submitted to KSeF and the PDF visualisation provided to the customer.

In practice, such differences may relate to:

• additional settlements;
• set-offs;
• previously outstanding receivables; or
• other information affecting the final amount payable.

The tax authorities may regard such discrepancies as the issue of two different invoices.

As a consequence, a business may become exposed to:

• an obligation to account for additional VAT;
• default interest; and
• fiscal penal liability.

For this reason, every business implementing KSeF should verify the consistency of data between its ERP system, the XML file and the generated PDF visualisation.

How should your business prepare for KSeF?

Implementing new software alone is not sufficient.

Without appropriate procedures, even the best system will not eliminate organisational errors.

It is advisable to conduct a review covering:

• invoice issuance procedures;
• correction procedures;
• advance payment settlements;
• employee purchases;
• XML and PDF consistency;
• document approval processes;
• ERP and accounting system configuration; and
• employee training.

These measures significantly reduce tax risks while improving the day-to-day efficiency of finance and accounting teams.

Summary

KSeF is not merely a technical compliance obligation. It represents a fundamental change in the way tax processes are managed within a business. The greatest challenges arise where internal procedures fail to keep pace with legislative developments and evolving tax authority practice.

Businesses should therefore regularly review their invoice issuance and correction processes, update internal procedures and monitor tax guidance relating to the operation of KSeF. Appropriate preparation helps minimise the risk of errors, improve the efficiency of finance functions and avoid costly tax consequences.

Do you need support with KSeF?

If you have any questions regarding the implementation of KSeF, the issuance of structured invoices or VAT compliance, please contact one of our tax advisers. We can assist you in reviewing your processes, developing internal procedures, verifying your systems’ compliance with the applicable regulations and ensuring that your business operates safely and efficiently within the new invoicing framework.