Tesco has been fined £300,000 for misleading customers after a single shopper complained about a national strawberry price promotion.
The case - brought by Birmingham City Council - related to the cost of 400g punnets of British strawberries sold at a store in the city.
The woman complained they were marked up as "half price" at £1.99 - with an apparent original cost of £3.99 crossed out.
She asked trading standards to investigate because she had never actually seen the original price imposed on customers and wondered if it was excessive and misleading.
The company admitted a number of offences under the Consumer Protection from Unfair Trading Regulations Act 2008 after failing at an earlier hearing to prevent the council bringing the case on a national basis.
Trading Standards told the court that Tesco's offer (at £1.99) fell foul of rules as it ran for 14 weeks though its original higher price strawberries (£3.99) were sold for a much shorter length of time.
The case also included a similar promotion when a pot of cream was included in the discount.
Birmingham City Council argued both offers were presented in a way that mislead or was likely to deceive the average consumer.
The judge described Tesco's turnover as a result of the promotions as "excessive" but agreed that while the supermarket chain had breached customers' trust, it was not the retailer's intention to deliberately mislead them.
In a statement Tesco said: "We apologise sincerely for this mistake, which was made in the summer of 2011.
"We sell over 40,000 products in our stores, with thousands on promotion at any one time, but even one mistake is one too many.
"Since then, to make sure this doesn’t happen again, we’ve given colleagues additional training and reminded them of their responsibilities to ensure we always adhere to the guidelines on pricing."