A statement from Visa said that a "temporary programming error at Visa Debit Processing Services caused some transactions to be inaccurately posted" and that the glitch "has been corrected and erroneous postings have been removed. Importantly, this incident had no financial impact on Visa prepaid cardholders. Visa regrets any inconvenience to our customers and has taken immediate steps to ensure this error doesn't occur again."
The statement said the glitch "impacted fewer than 13,000" transactions and a Visa official—Elvira Swanson, Visa's director of global corporate relations—said that those transactions impacted "about 13,000 consumers."
"It simply posted the wrong amount into online statements and it was corrected within hours," Swanson said, adding that the specific amount varied slightly from consumer to consumer, but that they were all "equally large amounts" and were "in some cases identical amounts."
As to what caused the glitch, that's a little trickier. Swanson initially said that the glitch "was part of an upgrade to the Visa debit processing platform" and it involved moving to a new file format, but she then E-mailed a short message saying—without elaboration or explanation—"the issue was not due to a new file format." It's unclear whether the original comment was indeed incorrect or merely unauthorized.