§6672 of the Internal Revenue Code imposes a 20% penalty on a taxpayer’s substantial understatement of income tax: (d) Substantial understatement of income tax (1) Substantial understatement (A) In general For purposes of this section, there is a substantial understatement of income tax for any taxable year if the amount of the understatement for the [...]

§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 [...]