In the last few weeks, in Utah, I have almost been overcharged several times at one of the big supermarket chains. In those cases, a product (cheese, bread, etc) was in a case with a price like $2.99, and in some cases even had a price sticker on it; but at the checkout line, I was charged a different price.
When I pointed out the discrepancy, I was charged the lower price with no questions asked.
I don't know if I am allowed to name the specific chain here, so I won't. I've also read in the newspaper about this happening.
Bottom line: pay attention at the checkout line. Put your cell phone away. It might save you some money.