In December 2017, Congress passed the Tax Cuts and Jobs Act which lowered income tax rates and, for many, put more money in their pocket. A good thing, right? Surprise! The IRS says 10 million Americans may have to pay tax penalties because of the tax act.

The Treasury Department was given authority to determine the new withholding schedule because the old one didn’t apply anymore. And because the tax act was passed so late in the year there wasn’t enough time to issue a new W-4. So, in 2018 you have millions of people who didn’t change their withholding instructions. Employers are withholding using the W-4’s they have on file. Therefore, taxes are withheld based on an outdated withholding system. And that’s where the rub comes in.

The Government Accountability Office (GAO) reviewed the revised federal tax withholding tables for 2018 implemented by the IRS and the Treasury Department. In their simulation, GAO discovered that 21 percent of workers are at risk of having their taxes under-withheld—3 million more than projections based on the old tax code. Only 6 percent of taxpayers are expected to have wages accurately withheld, while 73 percent are likely to have their taxes over-withheld. Accurate withholding was assumed to be within $100 of what is truly owed.

With its warning about potential tax penalties, the IRS is encouraging Americans to check their withholding status on paychecks, retirement accounts, pension checks and annuities, and social security checks—anything that adds to your taxable income. To determine if you need to pay additional taxes before the end of the year the IRS provides a tax withholding calculator on its website to help you figure out how much it will take to keep you in their good graces.

With 3 months left in the year, you may not be able to have your employer withhold enough to meet your tax liability. If that’s the case you can make additional payments directly to the IRS or increase your estimated tax payment. Retirees can request the Social Security Administration increase what’s withheld from SS checks.

Even if you think your tax withholding is just right, check the amount against the new tax code. It may save you headaches later on.