Well, some customers can be very absent minded.
In a case like this:
A customer bought a number of items at night. When they reached home, they just put their items aside. The next morning, they checked their items with their receipt. At the end of the list, there was an item which cost them RM5. But they couldn't find any other item which cost RM5. They already had an item which cost RM5. They suspected that the cashier had double charged them on that particular item.
So they came to the shop and complained that the cashier had overcharged them. But of course, it had to be confirmed. CCTV is the only way to do just that. After browsing the cctv, it was realised that the last item was something that the customer was holding. When counter checked with the customer, she realised that she had actually bought the item which cost RM5. Furthermore, the item was put aside in the house..
Just hope that all customers make sure that they have checked thoroughly before they complain. If over charging is really the case, the shopkeeper is ever willing to refund the customers..