If an IRS audit discovers unreported income, you may face penalties beyond just paying the additional tax owed. Common penalties include: 1) Accuracy-related penalty of 20% of the underpayment (see IRC §6662). 2) Delinquency penalties of 0.5% per month up to 25% for failure to pay on time (see IRC §6651(a)(2)). 3) Fraud penalty of 75% of the underpayment if fraud is established (see IRC §6663). However, the specific penalty rates for 2025 should be verified from IRS guidance like Revenue Procedure 2024-40. Reasonable cause may allow penalty relief in some cases.