Unbeknownst to you, while you innocently trot your respectable credit score around to lenders as you shop for a mortgage or car loan, your credit card companies may be mucking things up. Here's what's going on: A study by the Federal Reserve Board has found that some credit card companies are failing to report your credit limit to the credit bureaus that maintain your credit history and credit score -- and missing credit limits can often affect the interest rates you're offered when you need to borrow money.

Why would they do such a seemingly harmless thing? For competitive reasons. They don't want you to get "poached" by other credit card companies that routinely scan the system, looking for folks with certain characteristics. When your credit limit is omitted, your information is incomplete, and your score is lower than it otherwise would be, through no fault of your own.

This is a big deal. The credit card companies engaging in this nasty practice are not serving their customers well -- in fact, they're potentially costing customers tens of thousands of dollars apiece. For this they're being paid?

In a recent article, columnist Ken Harney offered some frightening details: "According to Fair Isaac, a 677 FICO score in today's market would qualify a borrower for a 6.23% 30-year fixed rate on a $150,000 home loan. A 30-point drop in that score because of non-reporting of credit limits would push the best rate available to 7.38%. Monthly principal and interest to the applicant with the artificially depressed score would be $115 a month higher than it should be." That's $1,380 per year in unnecessary payments -- ouch.

Lest you think that you're probably safe and this is happening to just a few people, think again. The Fed's study looked at the credit files of more than 300,000 people -- and discovered withheld credit limits on fully 46% of them.

Don't let yourself be a victim of this practice. Give your credit card companies a jingle and ask whether they're withholding your credit limit(s) from credit bureaus. Better still, don't just take the word of the person who answers the phone -- get a copy of your credit report and see for yourself how complete they are. Learn much more about your credit report and how to get a copy of it in our Credit Center.

Consider letting us know what you discovered on our Consumer Credit / Credit Cards discussion board. Let's see how the major credit card issuers, such as MBNA (NYSE:KRB), J. P. Morgan Chase (NYSE:JPM), Bank One, American Express (NYSE:AXP), Capital One Financial (NYSE:COF), and Citigroup (NYSE:C), stack up.

And by the way, if you're mired in credit card debt, don't despair -- you can get out of debt and build a beautiful credit score.

Longtime Fool contributor Selena Maranjian does not own shares of any companies mentioned in this article.