Two circumstances in which the IRS will waive the penalty for underpayment of estimated tax:
• In 2008 or 2009 the taxpayer retired after age 62 or became disabled, and the underpayment was due to a reasonable cause.
• The underpayment was due to a casualty, disaster, or other unusual circumstance, and it would be unfair to impose the penalty.

The new Form 2210 instructions for 2009 list a third reason in which the IRS will waive the penalty for underpayment of estimated tax:
• The underpayment was caused by adjustments made to income tax withholding tables that took effect in the spring of 2009.

The withholding tables were adjusted in the spring of 2009 to accommodate the new Making Work Pay credit.