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Asda's famous 'rollback' prices, for example, were found to have gone up rather than down in 30 cases, according to Watchdog while Tesco lured customers with "better value" bulk packs that were not always the cheapest option.
Similar findings by Which? recently prompted the consumer group to report Tesco (along with other retailers) to trading standards over their claims.
Solicitor and advertising law expert Andy Millmore told Watchdog: "When you look at claims that looks as though they're objective - like 'best value', 'better value', 'biggest box best value' - then the consumer is entitled to know that those claims are actually true.
"And if you deconstruct them and find that, in fact it's not the best value - then they're misleading and arguably that's a breach of the regulations."
Both Tesco and Asda said such price discrepancies were simple mistakes.
A Tesco spokesman said: "Our policy is absolutely to make sure that big packs offer better value and it is never our intention to feature promotions that do not offer real savings.
"With over 40,000 products and thousands of promotions, we do however occasionally make mistakes. When this happens, as it has here, we put it right and we are sorry."
Asda, meanwhile, blamed the 'rollback' error on a "glitch in our online system".
However, if you are heading off the weekly shop today, it might be worth checking that those oh-so-tempting offers are what they seem.
See the full Watchdog report on BBC1 at 8pm tonight.
Have you been caught out by supermarket price promises? Let us know below...