This has happened twice to one friend, and again tonight to another. They were both new to the area when it happened. It seems that if the woman behind the counter thinks she can get away with it, she'll pocket some of your money. about 200 pesos over this time. There's also a closed topic on this.
If you are new to the area, get a receipt and check it before you leave the store. If my spanish was a bit better, I'd go have it out with this lady.
Has this hapened to anyone else?