When an underpayment penalty is calculated on a tax return, this penalty is automatically added to the amount you owe on Form 1040, Line 76 (or Form 1040A, Line 45). If you are entering information on IRS Form 1040X, Line 15 Tax Paid With Original Return, you will want to keep in mind that the underpayment penalty may have already been included with the amount you originally owed/paid. You will want to make sure you subtract Form 1040, Line 77 from Line 76 (or Form 1040A, Line 46 from Line 45) before entering the amount on the Form 1040X.