Update: According to Polygon, Walmart has begun canceling pre-orders, claiming the posting went live in error and the system shouldn't be available for purchase yet. Original story as follows:
Nintendo fans, rejoice! Preorders for the highly-anticipated SNES Classic console have gone live online. As the follow-up to the popular (and horribly underproduced) NES Classic, the new console will feature 21 of the Super Nintendo's most popular games in a small HDMI-ready package.
Unfortunately, while GameSpot reports that while Walmart pre-orders went live on the 21st, the story was updated later that night with the news that the consoles had already sold out. Keep in mind that Walmart was the first company to begin selling pre-orders in the U.S., so this doesn't mean that all preorders have sold out; it's just this one retailer's available pre order stock that's gone. Thus far there's no word on when other retailers will open up pre orders or how many units Walmart had available for presale. The speed with which the consoles sold out may worry some gamers, though, since it seems frighteningly reminiscent of how difficult (or impossible) it was to get a NES Classic (for those who were able to get one at all).
Nintendo promised to produce more SNES Classic units than they did the NES Classic, but fans have been hesitant to assume that there would be enough of the consoles to go around come release. The quick sell-out suggests that those who want one aren't leaving anything to chance; future preorders will likely sell out too unless it starts looking like Nintendo is actually keeping its promise of increasing production to meet demand. Restocks of the NES Classic were sporadic when they happened at all, and Nintendo stopped production after one last restock to certain retailers. If the company follows suit with its new mini console, there's no guarantee that people who don't preorder will ever get one without having to pay a huge third-party markup.
Of course, that markup was part of the problem the first time around. Resellers were buying up as many consoles as they could, then selling them at places like Amazon or Ebay for sometimes double the retail price or higher. More likely than not, some Walmart pre-orders were made by resellers as well since the NES Classic and Nintendo Switch have proven just how lucrative reselling Nintendo's consoles can be.
With any luck, Nintendo has learned from its previous mistakes and has already planned for a large enough production push to meet demand. With claims that the console will only be produced through the end of 2017 and rumors already circulating about plans for an N64 Classic, Nintendo may have already moved on from planning for the SNES Classic's release and we could be looking at another release that simply won't meet demand.
The SNES Classic goes on sale on September 29, 2017.