This may come as a shock to some of you, but I'm going to say it anyway: people are not perfect. People make mistakes, and unfortunately, cashiers sometimes make mistakes when they ring up your purchases. As I was leaving the bread store today, I took a quick glance at my receipt and realized that the lady who rung me up had charged me five dollars instead of fifty cents for a loaf of bread (ouch). I went inside and got my refund, but many of us don't notice little mistakes like these and end up paying too much for something. Think about it: it takes 30-60 seconds to look at your receipt and five minutes at the most to get your refund, so you get $4.50 for 6 minutes of effort (a.k.a. 75 cents per minute for you math nerds).
Bottom line: check those receipts!!