I always shop with a list and record item prices and whether I have a coupon for the price. Before I leave Safeway or King Sooper, I always check my receipt (I'm sure people having to walk around me appreciate this). I used to find an error almost every time I shopped at Safeway but now just find the occasional error.

And to prove my coupon(s) if I do find an error, I send a copy of my online "list" to my iPhone. I can also check to make sure I'm buying the right product and size if there's a question when I get to the store.