Last year, about 4 percent of the Social Security numbers submitted by employers did not match the ones on file, The Christian Science Monitor reported. A 2006 study by the Social Security Administration inspector general found 17.8 million errors in the 435 million records in the SSA database.

A federal judge in San Francisco, ruling that "irreparable harm" would be done to legal employees fired because of mismatches, ordered the Department of Homeland Security to stop using number matching to detect illegal aliens. The department is appealing the decision.

Supporters of the DHS say 90 percent of the mismatches occur because an illegal alien is using a fake number. They also suggest legitimate workers can benefit from finding a clerical error early.

Sometimes the mismatches occur because of a name change as a result of marriage or divorce. In other cases, an employee, employer or government worker has made a clerical error.