Supermarket self checkout scam to be aware of

A new scam has happened few times recently at the local Waitrose. If the bill at the end seems too high, do check the first few items. People are starting off at a self-check till and putting through two or three items, then leaving the store with them without paying for them. These items stay on the till computer and get added on to the next person. There are checkers watching usually, but friends who haven’t checked have paid for something they haven’t bought.