DETROIT – This year you absolutely must fill out your federal income tax return before deciding how you’re going to spend your refund.

Why? Because you could be coming up short or even owing the government money, thanks to some quirkiness in a ”Making Work Pay” tax credit.

”It’s an odd computation,” said James Jenkins, president of a tax firm in Southfield, Mich.

Most likely to encounter some tax-time troubles are people who worked two or three jobs in 2009, two-paycheck couples, someone getting Social Security but also earning wages, and people who work but also have outside income.

Remember back in April when paychecks were tweaked to seem a tiny bit bigger? Millions of people started seeing a little extra money in their checks after withholding tables changed.

The American Recovery and Reinvestment Act of 2009 provides married couples filing jointly with a maximum $800 refundable tax credit under the Making Work Pay plan; singles receive a credit of up to $400. The credit applies to last year and this year.

Now, during tax time, many taxpayers will need to fill out a Schedule M to calculate the exact credit that they deserve under Making Work Pay.

There’s also a Government Retiree credit of up to $250 for people who received a pension or annuity payment in 2009 for service performed for the U.S. government or any state or local government if the service was not covered by Social Security.

But you cannot take the $250 credit if you and your spouse both received a $250 economic recovery payment in 2009.

The U.S. Treasury Office of the Inspector General for Tax Administration estimated that more than 1.2 million taxpayers may have to repay some or all of the Making Work Pay Credit and could be assessed the estimated tax penalty or an increased estimated tax penalty as a direct result of the credit.

The IRS estimates that only 68,000 taxpayers could be faced with an estimated tax penalty, which is easily waived for 2009 returns.

Taxpayers facing a headache this year might want to have extra money withheld in 2010.