I agree walmart doesnt normally sell defective stuff. Here is one thing that does happen though. In many areas they push the manufacturer to bring prices down so they change certain materials to accomidate it. It looks the same as the other product elsewhere but the numbers will be a little different. This is apparent in rifles (stock material is different on some), tents (floors of the eurekas are different), and fishing reels (use of more plastic than stainless steel in the guts). These are only the areas I know of. It normally doesnt effect performance enough to notice and normally willl outlast the length of time a normal person uses a product before buying a newer model.
Hope that helps some.