How can taxpayers protect themselves from incurring an underpayment penalty? 

When income is unpredictable, the tax return should be filed by the end of the year. 

Taxpayers should meet one of the safe harbor provisions for estimated tax payment requirements. 

The taxpayer should file an extension if they can NOT pay the taxes that are due. 

As long as the tax return is filed and taxes are paid by the due date, there is no underpayment penalty.