Super Mario 64 (Nintendo 64DD)

Super Mario 64 was originally going to be ported and updated for the Nintendo 64DD, a disk drive add-on for the Nintendo 64. The game was showcased at the Shoshinkai 1996 show[1]. However, with the add-on's failure, the Nintendo 64DD version of Super Mario 64 never released.

A version of the game was found[2], featuring a slightly altered castle theme and a few other alterations.

Reportings

 * Everything in this section should be taken with a grain of salt. These are all unverified claims.

Fourth Floor
On 4chan, an unverified claim was put onto /v/ back in 2006. This coincided with the release of New Super Mario Bros. on a discussion about Super Mario 64 DS. The post's integrity is uncertain, with the posts unarchived and no photographic evidence of this ever happening.

An anonymous user claimed to own a disk with "SM64" haphazardly scribbled onto it. One user suggested that this is the elusive Nintendo 64DD version of the game, and the user got to ask one of their abroad friends to see if they could snag a Nintendo 64DD from anywhere. Fortunately, they could, and were able to play the game.

Most of the documentation of what happened in the game has been lost, but apparently, the user ended up beating the game. After beating the game, a fourth-floor would open up for the player to access. This fourth floor housed new levels, which were very difficult. The contents of these levels have been lost to time, but it's known that the player had to fight a floating Wario head (bear a resemblance to the Wario Apparition).

If there is any other information about this anonymous user's gameplay, it should be put here.