Wal-Mart, the world's largest retailer, apologized for what it called a problem with a "third-party verifier's systems that had an inadvertent processing error."

Some customers said their gift cards worked after four or five tries but others told KXAS-TV, Fort Worth, Texas, their card never worked.

"The first time she scanned it, it didn't beep, and I thought, 'Hopefully, whoever gave this to me did really put money in there,' " shopper Mary Garcia said.

"We are working with the supplier to resolve the issue as quickly as possible, and we apologize for the inconvenience to our customers," Wal-Mart said in a statement Wednesday.