“A computer hardware problem caused more than 800,000 credit and debit card transactions to be double- or triple-billed last week at Wal-Mart stores nationwide, according to officials at First Data Corp., which handled the electronic payments.

The excess Visa and Mastercard charges, which occurred Wednesday and were posted on Thursday, have been reversed, First Data spokeswoman Staci Busby said Sunday.

Busby said the problem showed up on reports the company generates for quality control purposes. She said it’s unclear how many customers were affected, and that she had no other details about the hardware problem.

“Anyone who conducted a transaction with a Visa or Mastercard on March 31 should check their statements,” Busby said.”