Sainsbury's offer was circulated on the internet

The supermarket offered discount codes, which meant savings of �2-�12 on orders worth more than �60.

The offer's rules allowed only one code per purchase, but Sainsbury's website eroneously allowed some customers to use multiple codes, saving �43.

Sainsbury's said only some customers would get their orders.

Although "a handful" of orders had already been fulfilled, the supermarket said, other customers would be contacted by local stores and offered the choice of either paying the difference or cancelling the order.

'Frenzy'

Once the glitch, which allowed the multiple discounts, was discovered, it was widely circulated online, leading to hundreds of orders being made.

"There was a frenzy yesterday as people were attempting to place orders before the firm got wind of their mistake," one reader told BBC News in an e-mail.

Of the dozen or so people in his office who had made orders, he said, some had already been contacted by Sainsbury's - but others had had their orders fulfilled.

"Subsequently, (Sainsbury's) website has been corrected to prevent to use of multiple e-vouchers at once," the reader said.

Terms and conditions

Sainsbury's, however, insisted that the vouchers were sent to individuals in the post - and that the terms and conditions printed on the back of each voucher made clear that they were only to be used singly, and only by the person to whom they were sent.

For that reason, anyone using multiple codes would have had to amalgamate them from those sent to several different people, a Sainsbury's spokeswoman said.

The law's probably on their side... but from a PR perspective they may consider doing something else Gary Assim, retail law expert

The firm therefore had no legal obligation to honour those purchases.

"Our legal people say we're completely secure on this," she said.

A retail law expert told the BBC that in general internet orders were binding on a retailer as soon as confirmation was sent back to the shopper.

But, said Shoosmiths partner Gary Assim, there was an obligation on the consumer not to abuse an error.

In this case, he said, Sainsbury's could argue the consumer made the offer in bad faith.

"The law's probably on their side," he said, "but from a PR perspective they may consider doing something else."

Shopping glitches

The incident follows the widespread forwarding of a downloaded voucher from wine merchants Thresher, offering 40% off champagne.

Thresher's initially said the offer was intended only for the suppliers and their friends who were the original recipients, but later decided to honour all the orders made using the discount.

Other online shopping glitches have included the accidental pricing by Argos of a 21" television for �2.99 instead of �299.99, and Kodak's offer of a digital camera for �100 - instead of �329 - in 2002.