IRC Sec 24(g) includes a provision that where a taxpayer improperly claims the child tax credit (fraudulently or recklessly), a disallowance period applies during which no child credit is allowed. Where the improper claim is due to:

As part of these rules, the definition of underpayment has been modified to create a negative tax for when the underpayment is caused by a refundable credit. (IRC Sec 6664(a)). Consequently, an underpayment amount for purposes of the underpayment penalty calculation is created. In addition, the standard for the reasonableness exception to imposition of the underpayment penalty has been increased from "reasonable basis" to "reasonable cause." (IRC Sec 6676(a)).