She had really bought her VantageScore, which was jointly introduced this year by the three big national credit bureaus: Equifax, Experian and TransUnion. The VantageScore is based on a scale of 501 to 990. If any mortgage lenders use it, they don't talk about it. The woman had a VantageScore of 668, which made her think she was a prime mortgage customer. But it turned out that her FICO score was 574, casting her into the subprime category.

It's as if she turned on the radio, heard it was 32 degrees outside, put on a coat and stepped out, only to find that the temperature was 32 degrees Celsius -- about 90 degrees Fahrenheit.

Bad news for othersShe wasn't the only person who got a nasty surprise. Another Bankrate reader, who asks to be called "Bob from New Hampshire" because he doesn't want everyone to know his credit score, says he recently bought a VantageScore and it was 754. Soon after, he discovered that his FICO score was 681 -- a most unwelcome surprise. (He says he quickly boosted the FICO score 20 points after paying off a credit card.)

Bob feels that the credit bureaus are bilking consumers by selling the VantageScore. "If the score they're giving you here, you're paying hard money for, and it's not being used, what is the point?" he fumes. "If it doesn't have any practical value, they should disclose that upfront: 'This is not the widely used FICO score.'"

If you know where to look for them, you can buy the equivalent of FICO scores on the three bureaus' Web sites. TransUnion calls it a credit score, Equifax calls it a FICO score and Experian calls it a Plus Score.