If you’re counting on your tax return in 2010 for a vacation or to pay off some bills, you might be one of the 15.4 million taxpayers in for a shock when you discover that your refund will be lower than expected, or even worse, you might end up owing money to the IRS.

The Making Work Pay tax credit, part of the $787 billion economic stimulus package was passed earlier this year, reduced the amount of income tax withheld from taxpayer’s paychecks by up to $400 for individuals or $800 for couples.  The idea was that people with more money would, get this, buy more.

The problem though, announced in a report on Monday, was that the Federal Tax Tables used to determine the correct withholdings failed to take into account situations like taxpayers with two jobs, families where both spouses work, or taxpayers with Social Security income still in the workplace (the people who probably need their full refund the most).

The Inspector General of the Treasury Department in charge of tax administration estimated that over 10% of 2009 filers would owe additional taxes due to the complex and convoluted Making Work Pay credit.  To make matters worse, 65,000 of those people might also face penalties from the IRS for underpaying their taxes.

If you’re worried that you might be one of the Americans affected by this, visit the IRS withholding calculator.