Over the weekend, Walmart made the SNES Classic available for about 40 minutes before either the console's stock was out or Walmart pulled the pre-order. Either way, the page was soon unavailable.
Shortly after that, those that managed to get a pre-order in received notices that their orders had been canceled. It appears as though all pre-orders for the SNES Classic from last week will be canceled and that the pre-order page going live was the result of an error.
It should be noted that some folks are reported that Walmart has not canceled their SNES Classic pre-order.
When questioned on the canceled orders, a Walmart representative replied that orders had been canceled because the pre-order was "incorrectly published on the site on Friday night, 7/21." The representative went on to state that the SNES Class "is scheduled to be re-published next week."
If this Walmart employee is correct, you'll have another chance at grabbing the SNES Classic.