Staci Busby, a spokeswoman for First Data Corp., an electronic payments processor, said Sunday the excess charges on MasterCard and Visa accounts had been reversed. She said she did not know details of the hardware problem.
She was unable to say how many customers were affected, but did say Wal-Mart was the only retailer involved.
Anyone who conducted a transaction March 31 with a Visa or Mastercard at one of the chain's stores should check their statements, Busby said.
The errors were posted to customers' accounts on April 1. "First Data corrected the problem as soon as we discovered it," Busby said.
Busby said besides publicizing a toll-free hot line, Greenwood Village-based First Data was calling affected customers. She said some people may not see the reversed charges on their accounts until Tuesday.
Wal-Mart spokeswoman Danetta Thompson said cashiers were told about the mix-up and store signs were posted with First Data's toll-free number — 888-893-0626.