GamesIndustry International which says what the user found was "a mock up menu," and that it is now removed and inaccessible.
NeoGAF user Trike claimed to accidentally hack into a debug menu for Miiverse, simply by hitting X over the network's exit button. The user gained access to what Nintendo claims are mock-ups of the system's back-end. Among the things Trike stumbled upon were administrator lists and forum posts from developers. However, the info within his photos does resemble that of a mock menu, with repeated email addresses and messages.
Nintendo's official statement reads: "It has come to our attention that some people were able to access a mock up menu on Miiverse following the launch of Wii U in the US. Please note that this was only a mock up menu and has now been removed and is not accessible."
Interestingly, one of the user's photos shows a list of game-based subforums, with 'Yoshi's Land Wii U' and 'metal gear solid' among them. Of course, if the subforums are part of a prototype menu - one of them is for 'resident evils' - we can't draw anything from it.