In Ontario, the policy is: if the item scans at a higher price than the shelf tag or advertized price, you get that item at no cost--up to $10. If you have more than one, only one is 'free' but you get the others at the proper cost. Of course it doesn't matter where you picked up the item, the UPC has to match the shelf UPC. This is the biggest frustration for us (I work in retail) because no matter how much time we spend tidying, people tend to take an item off the shelf and just put it back anywhere.
I make a point of checking my receipt before I leave the store. Hate to have to haul the groceries back into the store to correct a mistake.