Most Common Income Tax Filing Mistakes and How to Avoid Them

Filing the income tax return can cause uncertainty, even for those who file year after year. A small oversight can lead to penalties, delayed refunds, or loss of tax benefits.

As a taxpayer, you are ultimately responsible for the accuracy and truthfulness of the information declared. At Adlanter, we want to help you identify the most common mistakes from a legal and practical perspective so you can fulfill your obligations with complete peace of mind.

1. Not checking personal and family information

It is common for personal information (name, ID, marital status, number of children) to be outdated. A simple error in this data can delay processing or affect family deductions.

✅ How to avoid it:

Before confirming the draft or submitting the return, make sure all your registry data is correct and, if there have been any changes during the fiscal year, notify the Tax Agency.

2. Forgetting to include income or applying deductions incorrectly

Many taxpayers forget to declare certain income (e.g., rental income or capital gains) or applicable deductions (donations, home investment, pension plans). This can result in penalties or unnecessary payments. Omitting income is one of the most serious violations. This behavior can be considered concealment and lead to a “failure to pay” tax offense.

✅ How to avoid it:

Keep an annual record of all your income sources and deductible expenses. Gather and keep all supporting documents (invoices, donation certificates, contracts, etc.) and review state and regional deductions you may be entitled to.

3. Confusing withholdings and prepayments

Sometimes IRPF withholding or prepayment information is misinterpreted, creating differences between what was declared and what was paid. This often leads to information requests from the Tax Agency to clarify the difference.

✅ How to avoid it:

Carefully review the withholding certificates issued by your payers (employer, SEPE, financial institutions). Calculate the withheld amounts correctly and compare them with your declaration.

4. Filing the return after the deadline

Missing the submission deadlines is a mistake with automatic consequences. If the result is a payment, filing late without prior notice from the Tax Agency incurs surcharges. If the Agency notifies you before submission, you will face a penalty.

✅ How to avoid it:

Mark the key dates in your calendar. The Tax Agency publishes the declaration schedule each year, and it is important not to leave it until the last moment.

5. Not reviewing pre-filled information

The Tax Agency provides a draft return, but blindly trusting it can be a mistake. Not all data is complete or correct. The IRPF law defines the draft as an assistance tool, not a definitive and automatically correct declaration.

✅ How to avoid it:

Carefully review each section of the draft and ensure all data matches your real situation. Check that it includes all your income, that deductions are correct, and that personal information reflects your situation as of December 31 of the fiscal year.

6. Not planning ahead

Filing without planning can lead to paying more taxes than necessary or missing important deductions.

✅ How to avoid it:

Organize your documentation throughout the year. Evaluate legal tax strategies that optimize your income and maximize refunds or benefits.

 

Key Dates | 2025-2026 Income Tax

 

Frequently Asked Questions

What documents do I need to file my income tax return?

You will need your ID, withholding certificates, proof of income and deductible expenses, information on properties, investments, and any other relevant tax documents.

Can I correct my declaration if I make a mistake?

Yes. The Tax Agency allows filing supplementary declarations or correcting errors within the established deadlines. The sooner you do it, the better.

What are the most common deductions that are often forgotten?

The most common include deductions for donations, main home investment, contributions to pension plans, childcare expenses, and certain educational costs.

What happens if I file after the deadline?

Surcharges and penalties are applied, increasing over time. Additionally, refunds are delayed, which may affect your cash flow.

Do I need a tax advisor to file my return?

Not always, but having a professional helps avoid mistakes, maximize deductions, and ensures peace of mind. At Adlanter, we manage the entire process so there are no surprises.

How Adlanter Can Help You

Through Adlanter’s accounting and tax advisory, we assist individuals and businesses in fully managing their income tax return, avoiding mistakes, optimizing deductions, and ensuring every procedure is completed correctly and on time. Our expert team reviews every detail and advises you on any tax question.

Don’t leave your income tax to chance: with planning and professional support, your declaration can be fast, secure, and profitable. Contact us without obligation.