We're only in the second week of tax season — and some filers are already in for a nasty surprise from the IRS.
This spring marks the first time taxpayers are submitting returns under the Tax Cuts and Jobs Act, which went into effect in last year.
Though the 2018 filing season only started on Jan. 28, some early filers are discovering that they either owe the IRS or they'll be getting a smaller-than-expected refund from the taxman.
They took to Twitter to air their grievances.
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These taxpayers aren't alone, either.
About 30 million people, or 21 percent of U.S. taxpayers, are expected to owe money to the IRS this tax season, according to a report from the Government Accountability Office, a legislative agency that provides data to Congress.
"The people who are most likely to be surprised this year are the ones who lost some deductions they had last year and who didn't make changes to their withholding," said Nathan Rigney, lead tax research analyst at the Tax Institute at H&R Block.
Here's who might expect a tax bill or a smaller refund this spring and why.
New withholding tables 
In 2018, the Treasury Department and the IRS updated the withholding tables to reflect the new Tax Cuts and Jobs Act.
Major changes from the new law include the end of personal exemptions, the doubling of the standard deduction, and reduced individual income tax rates.
These withholding tables are guidelines that your employer follows in order to deduct the right amount of income tax from your paycheck.
Mnuchin discusses tax withholding tables at White House briefing 
The tables are intended to work alongside Form W-4, which you can use to tailor your taxes based on whether your spouse works, whether you have children and other factors.
Withhold too much, and you get a refund in the following year — but you've given the government an interest-free loan.
If you fail to withhold enough taxes, you'll see more cash in your paycheck in the immediate term, but you'll owe the IRS the following year.
The tax agency has been reminding taxpayers through 2018 to take a second look at their withholding and make estimated payments, if needed.
Even retirees were encouraged to ensure they were withholding sufficient tax from pension and Social Security payments.
Last month, the IRS announced it would waive the estimated tax penalty for filers who paid at least 85 percent of what they owed during 2018.
Normally, you have to pay at least 90 percent of your tax liability in order to avoid the penalty.
Watch your back
