Some Tesco shoppers ended up paying too much for their groceries thanks to a glitch with the Clubcard loyalty system.
Shoppers found that discounts that should be applied for anyone using the Clubcard weren't being taken off their final bill.
The discounts appeared not to work for some people using click and collect and deliveries.
The supermarket said the problem, which was first spotted by Love Money only affects a "small number" of users.
According to The Mirror thousands of shoppers will have been hit by the bug.
One shopper tweeted: "Hi @Tesco I’ve just had my shop delivered but I’m trying to work out how to get the Clubcard prices? These haven’t been knocked off my bill and my Clubcard is registered with it. Can you help?"
Another said: "I didn't get Clubcard prices on my online shop I collected today... charged full price!"
Tesco said all affected customers have been emailed and provided with a £10 off e-voucher.
It added that the issue only affected online purchases and was fixed a few days ago.
Tesco Clubcard Prices, launched last month, gives shoppers using the loyalty card a discount on their shopping.
Money should be taken off hundreds of brand-name products with savings of 10% to 50%.
When you order a food shop online, your payment will usually be pending until the items arrive.
It means customers who have been charged more - or less - won't find out how much they've been billed until the day of - or after - their delivery.
