My son once did the weekly shop and had bought about the same amount of things as usual. He told the assistant, and was robbed off. He paid the bill, moved away from the till, and then checked the receipt. He had been charges £99 for something that should have been 99p. Waited for the assistant to finish with the customer who was being served, then told the lad about it. No apology, just told "go to the customer care desk". He then had to wait in a long queue, then a further wait while an investigation went on, finally getting the money back with no apology whatsoever. Ever since then i have always checked receipts, and have found mistakes lots of times. Varies from being charged twice for something, charged full price for things which are on special offer, and and twice charged for something that was rung up twice instead of once, then again when the cancellation went through as another charge. Very peculiar that I have never got a receipt which undercharged me!