This is older info, so it might not be up to date.
Wal-Mart gets its Card Supplies on Tuesday and puts them out Wednesday. So sometimes, the person in that area will put up new stuff because they do not know/care about the actual release date. This happened a lot with Yugioh (especially at the store I went to because the person who was in charge of the area was my Dad's (now ex) girlfriend) and no one there cared about cards. They contacted me if they needed to know something...and that did not happen often.
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And vendor or employee error? Don't matter. They rung up at the register.
Most things at Walmart don't have a street date built into the PoS system unless they're also under lock and key. Otherwise people will just turn around and steal the thing instead of buying it. They only pay the penalty if they get caught.
(p.s. take your receipt and send the info to Wizards. They'll want the store number that's listed)