Wal-Mart found the errors in 2004 during a routine internal review, [John Simley] said. Problems stemmed from "calculation errors and procedural errors" at headquarters, he said, but he declined to say how they occurred. Wal-Mart reviewed data going back to Feb. 1, 2002, the first year for which it has "consistently reliable records," Simley said.

Wal-Mart said it found that some non-exempt salaried employees were expected to work 45 to 48 hours before they earned overtime pay. They did not receive the extra half-time they were owed for hours between 40 and 45 or 48. Other errors also occurred, Wal-Mart said.

Wal-Mart has been involved in other overtime suits. In October, a Pennsylvania jury decided that Wal-Mart forced hourly employees to work through breaks and beyond their shifts without overtime pay. The plaintiffs were awarded $78.5 million. Wal-Mart is appealing.