A gigantic data-chomping computer burped last month, giving hundreds of retailers and customers in the Washington area and nationwide a $500,000 case of financial indigestion.

On July 28, Visa's main computer system in California billed some customers here and across the United States twice for the same credit card purchases.

A $30 restaurant tab became a fancy $60 affair. A nice $50 sweater got a whole lot nicer at $100. And with that $1,000 camcorder -- voila` -- you're way over your limit.

"It was an unusual and rare processing error that was detected within hours," said Visa spokesman Gregory Holmes. "We deeply regret any inconvenience to those merchants or any card holder affected and have taken necessary steps to make sure that these types of processing errors does not happen in future."

According to Holmes, the "glitchees" included Visa and MasterCard, which also uses the Visa computer facility, as well as 50 banks and at least 200 merchants.

The problem occurred after merchants sent their daily charge tallies electronically to Visa to be sorted and sent to the correct member banks that issue the cards and bill customers.

But a hardware malfunction at Visa's San Mateo facility, which processes hundreds of millions of dollars in credit card charges daily, transmitted some data twice to banks.

A monitoring system at Visa picked up the error quickly and began calling banks about the problem, but not before some banks charged customers, who then quickly called back the merchants in this snake-eats-its-tail scenario.

"It's upsetting that merchants are the ones that take it in the neck for their screw-ups, since the customers then think that we are trying to rip them off," said Michael Schneider of Bethesda's Back In Shape, which sells orthopedic furniture and related goods.

Since Visa never publicly acknowledged the foul-up, Schneider had to call all of his customers about the problem, a task made harder by new laws that prohibit stores from recording telephone numbers on charge slips.

"All they needed to do was put out a press release and bring the customers out of the dark about the whole thing," said Schneider.

One retailer in New Jersey wasn't even aware of the problem until he was balancing his store's accounts and found he had a $2,000 windfall. "If I make a mistake, I tell people right away, especially if it's about money," the store owner said. "But I have not heard from them at all."

But, said Visa officials, it's much ado about not much, since customers were not billed for any finance charges and will be fully credited by Visa, which services 8 million merchants and has 22,000 member banks worldwide.