Fiscal-Cliff Fallout: How the AMT Would Hit Taxpayers

Without Congressional action, tens of millions would see dramatic tax increase.

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[Read: Avoid These 10 Common Tax Mistakes.]

If no solution is found, the 2012 tax season will be a mess, according to Miller. In his letter to Hatch, the acting IRS commissioner said the agency did not reprogram its computer systems to account for the AMT.

If Congress passes an AMT patch, "The IRS would likely be able to open the 2013 tax-filing season with minimal delays for most taxpayers," Miller wrote. He then warned: "However, if there is no AMT patch enacted by the end of the year, there would be serious repercussions for taxpayers."