A lot of lines have price alterations which take place on a Monday and the majority of shelf labels will be changed but there will always be an odd few which get overlooked. At the checkout , the system will give you the best deal and if it doesn't just complain.
Last week after getting home after doing my weekly shop at Tesco, I realised I must have left my pack of four recycled-paper bog rolls at the checkout
I went back next day to the customer service desk, half expecting the cold shoulder, but after presenting my receipt and explaining the situation, was invited to go and get a replacement pack from the shelf, no quibble, no hassle