Consumers who spotted that for every milkshake they bought Tesco refunded double the price have milked the offer to bag bargains on other purchases.
The supermarket chain advertised a buy one get one free offer for customers who bought six Frijj milkshakes.
But at self service tills at some Tesco Express stores the offer became buy one get two free.
The glitch not only gave customers a free bottle of milkshake but refunded £1.25 from the rest of their shopping bill for every milkshake they bought.
Thrilled consumers swapped news of the offer on the Hot UK Deals and Money Saving Expert forums.
BargainLovingMummy explained in a post to Money Saving Expert users: "You need to buy in multiples of six and with other items - as you need to take the total to a positive figure."
Taking her advice, user cidergirl posted: "Thanks - just been and got six bottles and bottle of ribena for 8p."
User jtz wrote: "Worked for me, bought 18 - phoned home to ask someone to help me carry them. They didn't come!"
One bargain hunter said they had bought 12 milkshakes and a tin of Quality Street for £2.30 while another claimed to have bought 12 chocolate milkshakes and three two-litre bottles of Pepsi for 21p.
A Tesco spokeswoman said: "My understanding is that it was a problem on Friday and it was resolved then."
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