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Getting your taxes filed takes a weight off your shoulders, but that relief can turn to dread if you realize you sent the IRS bad information. Maybe you overlooked a form from your bank with interest information, or you didn't realize that hosting travelers on Airbnb might have to be declared as rental income, or you completely forgot that you donated an old car to charity. Now what?

"If the return has already been accepted by the IRS, then your only alternative is to go back and file an amended return," says Craig Wild, a CPA with Wild, Maney & Resnick, LLP. Unfortunately, an amended individual return can’t be filed electronically, so you'll have to file a hard copy.

You'll also need to pay any additional taxes you owe as a result, so if you catch the mistake by Tax Day (that's April 18 this year), Wild recommends estimating how much you'll owe after the second round of number-crunching (you could use a tool like the IRS's estimation calculator here) and sending the IRS a check. "If you pay your taxes after the due date interest and penalties will accrue," he warns. If you're owed money, you have three years to file an amended return, but if you owe, file as soon as possible so you don't run up interest and fees.

"You’re going to want to amend the state [return] as well," says Michele Schlereth, CPA and senior tax manager at Baker Tilly. Do them both at once, because if the information the state gets from you—the old return—is different from the amended return it gets from the IRS, your state tax officials are probably going to have some questions for you.

Contrary to popular belief, tax experts say filing an amended return won't make you any more likely to get audited, but keep in mind that outsized deductions in general—whether on an initial or an amended return—can be a red flag. Miss a W-2? That might not raise many eyebrows, Schlereth says, since withholdings were already taken out of your paycheck over the course of the year. If you missed a 1099—say, earned income from a side business or gig-economy participation like driving for Uber—that might get a closer look. If you're amending your return to increase unreimbursed business expenses—well, consider how badly you need those deductions before re-filing. If fixing the mistake would mean a larger refund, the experts say you'll need to determine if any fees you pay a tax preparer to file an amended tax return will offset the extra money coming back to you.

On the other hand, if the mistake was an honest oversight or something out of your control (like not getting a form from an employer or financial institution until after you filed), it's perfectly okay to explain the extenuating circumstances that prompted the amended return, Schlereth says. "Tell them you realized it was a mistake you took an immediate action," she says. "There’s a place to put an explanation of what happened," so take advantage of it.