Each year, about three million people overpay their federal income taxes but don't file returns. Taxpayers usually have three years to claim a refund. When a non-filer waits more than two years before seeking a refund, the IRS often seeks more tax because the taxpayer has not filed. If the taxpayer appeals to the Tax Court to avoid paying the additional tax, the IRS says the refund period is only two years. Now the Supreme Court decides if a deficiency notice can shorten the time to claim a refund in the Tax Court.