[CORattler]

Of course systems can be modified/programmed to perform all kinds of tasks including phony charges, higher tax rates, etc, etc. When I go to Home Depot I just show my military ID and the cashier keys in a sequence that gives me 10% off the total charge (imagine that). After all, most registers are either networked computers or "dumb terminals" connected to a centralized computer or server.
I don't believe that a business as big as Walmart with so much to lose, would intentionally scam customers in this manner or tolerate any employee who would do it (no, I'm not saying all of their employees are saints). It would be just as foolish for any other business ... the negative publicity would be ruinous. Does it happen at all? Possibly. There are jails full of people who thought they could "get away with it".
I think it's a good practice to check your receipt against items purchased. We do it all the time.
The posts generated as a result of the OP have highlighted the need for awareness and that's good... hoaxes and internet myths, however, are a waste of bandwidth.
All this is, of course, JMHO. No offense intended to anyone on the MOC.