The SII enforces tax law strictly and is notoriously thorough and accurate with its accounting.  If you make a mistake with your accounting and the SII believes it was made in good faith, they have, by law, 3 years to investigate and penalize you; if they believe you attempted fraud, they have 6 years.

There is a 0.5% interest charged on unpaid taxes, up to 300% of your original liability.  If the tax evasion is determined to be fraud, you will likely go to prison.

If the SII is investigating you for potential tax evasion, even if it was a simple mistake made in good faith, we highly recommend that you hire a tax lawyer to assist you.