Right about now you should begin to notice a little extra money in your pension or paycheck. It’s the result of the American Recovery and Reinvestment Act, the fancy official name of what is more commonly called the federal stimulus package.

If you don’t have a pension or a job, and you receive Social Security, you’ll get your shot of stimulus money around the end of May from the Social Security Administration in the form of a separate, one-time payment of $250.

The idea was to put a little extra money in everyone’s pocket to spend, thereby helping the economy to recover. That’s a good thing, and I certainly intend to do my part! However, for some taxpayers there’s a catch that could result in an unpleasant surprise next year at tax time.

The federal government chose to make the extra stimulus money available by telling employers to withhold less federal tax from their workers—up to $400 for individuals and $800 for couples filing jointly. Recently the Internal Revenue Service said the reduction also applied to monthly pension checks.

As you know, your employer (or pension plan) withholds a portion of your check for state and federal tax, based on factors such as how many allowances for dependents you want to take. The more allowances you take, the less is withheld in taxes. Generally speaking, if you find yourself at tax time with just about the same amount withheld as your tax bill, you’re doing well.

But the Pension Rights Center, a consumer organization dedicated to promoting retirement security, recently pointed out that some folks could be headed for an unexpected tax bill in April 2010.

Here’s the reason. If your employer reduced your withholding, but your tax obligation remained the same, you would owe more money at tax time than the amount that had been withheld. To make sure that didn’t happen, Congress created the Making Work Pay tax credit, which is intended to offset the amount by which your withholding is reduced. If your withholding is reduced by $400 and you receive a $400 tax credit, no problem.

But some filers aren’t eligible for the tax credit—for example, retirees who have taxes withheld from their pension but don’t have any earned income. That means that next year, when they file their taxes, they’re going to have to make up for the fact that not enough was withheld from their check.

“Our biggest concern is for retirees who are possibly living from pension check to pension check,” said Rebecca Davis, staff attorney for the Pension Rights Center.

Davis and Pension Rights Center director Karen Ferguson wrote Treasury Secretary Timothy Geithner asking him to direct the IRS to clarify that the new withholding tables do not apply to traditional employer-sponsored pension plans. That would keep the employers from withholding too much from pension checks. So far, Geithner has not responded.

Who else might be at risk? People who receive pensions and also have a job might come up short at tax time if both their employer and pension plan withhold less tax. People who hold two jobs also might end up with twice as much withheld from their pay as they are entitled to as a tax credit.

Fortunately, there’s a way for individual taxpayers to fix this. The IRS website has a calculator and a W-4 form to fill out to change your withholding, in order to make sure you don’t have to scramble to come up with extra money next year.

Unfortunately, I think it’s likely that few taxpayers will understand the potential danger of owing more money than expected at tax time next spring. I probably pay more attention to tax and retirement issues than most of my friends, and I didn’t know about it until I read the letter from the Pension Rights Center. But for those who don’t get the word, it could be tough. They could, in the words of Davis and Ferguson, “face an unpleasant surprise when they file their 2009 tax returns and learn that insufficient funds were withheld to cover their tax liabilities. This could even subject some to penalties.” Yipes!

Martha M. Hamilton, formerly with the Washington Post, writes a regular column, Your Financial Future, for AARP Bulletin Today.