Caveat emptor, indeed!
When, I wonder, does incompetence in this area cross the line to the legally actionable -- especially given that WalMart has a reputation for being particularly unpleasant with customers who insist that such errors be corrected, an attitude that leads logically to laxity if not outright intentionality of these discrepancies being part of their corporate culture. Certainly there has to be a class action in something so pervasive -- but that's too easy and, for crooks of the WalMart kind, probably not enough to prove salutary. No, this is the sort of thing that a courageous, creative prosecutor -- or few hundred, pursuing things as far up the corporate chain of irresponsibility as can be traveled, should tackle.
In the meantime, check your area for uniform pricing ordinances/statutes. These typically require that you be charged the lower of an item's marked price or actual price and that if you are overcharged, that the offender pay ten times the difference between the marked price and price charged as a penalty. If your area lacks such an ordinance or statute, ask your councillor/ alderperson/ commissioner/ legislator to author one.