Some taxpayers aren’t paying enough attention to the tax they owe. A recent IRS Fact Sheet showed that the number of taxpayers who pay penalties for underpayment of estimated taxes jumped 39% from 7.2 million people in 2010 to 10 million in 2015. Reasons might include miscalculation, forgotten events such as the sale of a capital asset, or unexpected exposure to the 3.8% net investment surtax. To stave off any penalties even this late in the year, you should increase withholding, if possible. Boosting estimated payments will mitigate but not avoid all penalties.