I was at my local supermarket "Super 1" and caught an error at the cash register which the cashier corrected. On the way out, I was looking over my receipt and found another mistake. I was charged full price for a sale item. I went back to the customer service counter and I was given back the full cost of the item
I was told by 2 persons at the customer service counter that the scanning person was out sick so mistakes happen. I said back to one that perhaps they should have a sign that warned people to be extra careful of their prices on days without this person. Also, the store assistant manager told me that the store's policy is to give you the item for free should they charge you a wrong price. This surprised me because I had never been given the full price back for any of their previous mistakes.
So, it pays to check your prices and find out the store's policy from management.