FYI: If you have a problem with a particular POS reader at Walmart (or Neighborhood Market by WM) like I did today, the cashier can suspend the transaction, print the receipt and another cashier can retrieve your check out transaction and you can pay at a POS reader that hopefully works.
Just my luck, I used a terminal that acknowledged the card, but froze with a white screen after the "MasterCard... Do Not Remove Card" message. I got this using two different Chip cards so I knew the problem was the reader. I also got lucky that there was a cashier that was on the ball and knew her stuff.