Check your sales receipts
I just saw an investigative piece on a local news show where they went to various stores and bought things to see if they scanned correctly.
They found scanning errors everywhere they went... J.C. Penney's, Target, Macy's, to name a few. They said some stores were charging only pennies more and others quite a bit more than that. I believe it was Target that has been overcharging so consistently they actually got fined. If you buy a lot of things, and if you are using your credit card, it's kind of easy to miss an overcharge while you're at the check-out.
Last summer I went to Lowe's to pick up a few things and after I got home discovered I had been charged $19.99 for hardware cloth that I didn't buy. DH went to our local Dollar Store and was charged for $20 cash back that he didn't get. (In that case I think there was some out-and-out thievery going on by the sales clerk but he gave her the benefit of the doubt when he went back to get it straightened out.) And Lowe's took the charge off by phone when I called.
But the point is, if you're someone who just throws the receipt in the trash after you get home, you might want to reconsider that. Maybe you didn't get the good deal that you thought you got.