I always double check everything that scans on their screens and what my receipt says before I leave, I've got very good at catching mistakes that don't ring up on the screen properly. One of our local stores will actually give you the item for free if they screw up on the checkout. Especially in clothing stores too on those we will do an additional mark down of 25% at the checkout, most of the time it doesn't do it and they have to enter it in on the register.