Second time in three weeks I have noticed an overcharge at the checkout, this morning it was sugar, ticket price 70p checkout £1.09, the other week it was apples overcharged by 50p. They used to give you back double the difference but not anymore so I presume it is a common occurrence now probably due to less staff. When I see shoppers with full trolleys I wonder if they ever check prices and imagine Tesco and probably other supermarkets do quite well out of these mistakes before someone brings it to their attention.
I do notice that sometimes the discounted price does not flash up on the till, but then appears as a credit further down the receipt. E.g. I bought some vodka that was £17 reduced to £12. It initially flashed up (scanned) as £17 so I got in a rage, then my partner pointed to the screen and showed me the line item that said something like "vodka deal -£4". I wouldn't have noticed it because it didn't beep.
It is all a con though; where we save in some ways through deals or loyalty cards, we expend in other secret ways.
I haven't noticed over-charging though, but you are right, it only takes 1 member of staff (or computer) not to enter in the correct code and probably hundredds of people a day are getting over charged because they don't check. I sort of check as I'm scanning, but I don't study the receipt afterwards. I just have a mis-guided trust for the shop and hope they've got it right