Bastards did a similar thing to my wife one year at Christmas, except that the item they scanned in for $50 wasn't even hers. Turned out it was a toy that cashier had under the counter and scanned it in along with my wife's items (and then stuck it back under the counter). Thank goodness she had kept a good idea of what her running total should be and the alarm in her head went off when it was $50 too high. Then the cashier acted like she was ignorant of the whole thing. CS acted like it was no big deal either. We got our $50 back, and I guess some other poor soul paid for her kids toy, because I doubt the CS manager did anything about it.
I must admit that I have not stopped using "Walton's wallet vacuum service" completely, but they don't get near the money out of me as they used to. I try to keep my dollars locally if I can......but there are a very few limited items that you can't get anywhere else in town, so I have to get it there.