§6672 of the Internal Revenue Code imposes a 20% penalty for negligent underpayments of tax:

(a) Imposition of penalty

If this section applies to any portion of an underpayment of tax required to be shown on a return, there shall be added to the tax an amount equal to 20 percent of the portion of the underpayment to which this section applies.

(b) Portion of underpayment to which section applies

This section shall apply to the portion of any underpayment which is attributable to 1 or more of the following:

(1) Negligence or disregard of rules or regulations.

(2) Any substantial understatement of income tax.

(3) Any substantial valuation misstatement under chapter 1.

(4) Any substantial overstatement of pension liabilities.

(5) Any substantial estate or gift tax valuation understatement.

This section shall not apply to any portion of an underpayment on which a penalty is imposed under section 6663. Except as provided in paragraph (1) or (2)(B) of section 6662A (e), this section shall not apply to the portion of any underpayment which is attributable to a reportable transaction understatement on which a penalty is imposed under section 6662A.

(c) Negligence

For purposes of this section, the term “negligence” includes any failure to make a reasonable attempt to comply with the provisions of this title, and the term “disregard” includes any careless, reckless, or intentional disregard.