Hi yelenots,
I usually shop at Tescos as it's most convenient for parking etc. I've always been in the habit of mentally adding up my purchases from when I was in a real financial mess many years ago! I always know what my bill will be at the checkout within a pound or so, and often exactly. I've always been surprised how often I'm overcharged at the checkout - and it's never in my favour! I've always estimated on an average trolley of £40 that I'm overcharged once every four visits by an average of £5. I've been overcharged between £5 - £10 on quite a few occasions, and just occasionally over £10.
The price differences come from those marked on the shelf being different to those at the checkout. When I query the bill the staff go and check the shelf price then come back and say it wasn't put on the computer correctly. I often find this with reductions and offers such as Buy One, Get One Free or Half Price, but the checkout charges full price for both items. Reduced clothes are another example where the checkout ignores the reduction and charges full price.
The number of times I see people in front of me never checking the amount at the checkout. They just wave a card, sign their name and wander off. So many must walk away never realising they have just paid much more for their shopping than they should have. I think supermarkets must make so much money that way.
Maybe next time you go to Asda yelenots, take a pencil and make a point of adding the shelf prices up so you have a good idea of the amount you should pay before you checkout. I think you'll be surprised.