According to a NeoGAF post, user "Trike" was trying to set up his Miiverse friends list after purchasing a Wii U, and accidentally came across what appears to be the console's debug menu.
Screenshots posted by the user show how he was able to access a list of admins, with options to regenerate admin passwords and delete admin accounts. According to a follow-up post, he was also able to view messages from developers.
Another NeoGAF post by user "Tom_Cody" reported a similar problem. The user posted photos showing a test screen, which reportedly appeared when he tried to sync his Wii U GamePad with the system.