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.

Indeed, the average refund for the week ending Feb. 1 — the first week of the new tax season — was $1,865, according to data from the IRS. In comparison, the average refund issued for the year-ago period was $2,035.

Some taxpayers 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.