Dummied Out: The beta version of the game (which had the working title Super Mario Kart R) had an art style more akin to Super Mario RPG, still had the Feather item from the first game, and one of the racers was a Magikoopa (presumably intended to be Kamek). In the release version, the art style changed to that of Super Mario 64, DK replaced Magikoopa and the feather was considered a Game Breaker.
Fan Nickname: The Spiny Shell makes its debut here, known by fans as the Blue Shell.
Urban Legend of Zelda: It was a fairly common belief that the Green Thwomp in Bowser's Castle was a secret racer.
Entering Peach's Castle in Royal Raceway. In reality, this is just where the award ceremony takes place. When the track was revealed to be a retro track in Mario Kart 8, the castle is shown to be blocked off, which may or may not be Ascended Fanon