After Dec. 31, there’s nothing you can do to affect your taxes for the year that just passed, right? Not so. There are many things you can do before you file to screw up your return. We’re talking about the filing blunders that can increase your tax bill, raise the odds of unwanted IRS attention, or cost you if you get audited. Leaving aside obvious ones, such as forgetting to sign your return, here are seven gaffes, or missed opportunities, I see most often:

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