List of Mario Kart 64 pre-release and unused content
From the Super Mario Wiki
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This is a list of pre-release and unused content for the game Mario Kart 64.
The working title of this game was Super Mario Kart R (the "R" stands for "Rendered"). Kamek was originally intended to be one of the playable characters, but ended up being replaced by Donkey Kong. The Character Select screen was also different, the characters faced the player, and Kamek can be seen in Donkey Kong's space. Boos from Banshee Boardwalk had also a different look, the HUD was different from the final version, and item boxes were also completely black with colored question marks on them. The Cape Feather, which was in Super Mario Kart, was also intended to be included, as seen in a certain screenshot of Super Mario Kart R. This particular screenshot can been seen on the back of the packaging of the Nintendo 64 system.
There is a debug menu that remains in the game's coding. It has a few more options than the regular gameplay, and some are unused. It can be enabled with the Gameshark code