As the till areas are so small, 'some' assistants will bag the goods over the scanner once they have been scanned. Problem is, as the the bags are so thin, the scanner can pick up items that are already in the bag after they have been scanned, therefore you are buying them twice, or in my case, three times!
If I hadn't checked the receipt, I'd have paid for three lucozades when I only actually purchased one.
This has now happened to me on three occasions now, and the last time (yesterday) I managed to stop the guy mid transaction.
Next time you go into a Tesco Metro, make sure they haven't got the baskets on the till surface that only gives them the scanner area to work with.
You've been warned!