Shelf and check-out prices different

Guys, check your till slips before you leave the shop. In the past week, i.e. since Monday, I've been overcharged compared to the marked price on the shelf on three occasions. Once, R11, then R2 and today R1. Now, you may think who's going to bust a gusset for R1, but it is the principle. Please check you slips before you leave the supermarket.