An unused Mission Mode, similar to that of Mario Kart DS, was discovered in the code of Mario Kart Wii, nearly a decade after its release. The unused feature is missing key bits of code, such as mission parameters and menu text, to properly work without manually adding it in.
Every year, the American broadcasting network Nickelodeon organizes a survey asking children about their favorite video games. Mario Kart Wii won gold prize three years in a row (2008 - 2010).
There is an unused coin graphic found in the game files.
There is an unused graphic of Mario in a kart which may have been used for the banner icon on the Wii menu.
It's possible for Lakitu to be crushed briefly by obstacles when informing racer's that they're heading the wrong way. This can be done by the Thwomps in Bowser's Castle and the Crushers in Toad's Factory.
A graphic of the American flag can be found in the game's files. This could suggest that there was originally going to be a language selection screen.
The advertisements in Coconut Mall are filled with Miis from the Wii's memory. Most customers in the mall are also Miis from the Wii's memory.
In the course "Moonview Highway", billboards can be seen for "The Mushroom Moon", featuring Petie Piranha climbing a building. This may be a reference to the famous "King Kong" movie poster.
In the course Dry Dry Ruins, the Yoshi Sphinx is changed to have a Mii's face if the player is using a Mii.
Rainbow Road has several features tied to Super Mario Galaxy. Star Bits will appear on the course if there is a Super Mario Galaxy save file on the console, and part of the background music is sampled from the theme of the Good Egg Galaxy.
The Blue Falcon, normally piloted by Captain Falcon in the F-Zero game series, is an unlockable kart in Mario Kart Wii. It is available to all small characters, but despite the name "Blue Falcon", it is only blue when driven by Baby Mario, Toad or a Male Mii.
The Mach Bike, a medium sized bike, is based on the bike from the NES game Mach Rider.
Certain names, such as "Hitler", are banned from online game modes. Attempting to connect with a "forbidden" name will instead display a message stating that it must be changed to proceed.
Petey Piranha, Paratroopa, and a Hammer Brother were all going to be playable in the game, as well as a third costume for Miis. The Chain Chomp item was going to return, but was replaced by the Bullet Bill in the final game. Several graphics supporting this appear within the game's data.