In Mario Kart 64's Bowser's Castle, there is what appears to be a green Thwomp locked up in a jail cell. This Thwomp is not actually green, but simply lit in a way that makes him appear as such.

This Thwomp would inspire urban legends claiming that players could bail him out through varying elaborate tricks, after which he would be playable under the name "Marty" (likely chosen for the ease of editing hoaxed images with the names Mario, Toad, and Yoshi)

When N64 Bowser's Castle returned in Mario Kart Wii, "Marty" would be made the color of the rest of the Thwomps, suggesting that his green complexion was simply an error and not an intentional design choice.

All the characters that return in Mario Kart Wii reuse their Mario Kart: Double Dash!! models, except for the newcomers and Dry Bones, who debuted in Mario Kart DS.

Donkey Kong’s mouth texture is placed on the outermost toes of his feet. This may have been just a mapping error.

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.

There are 3 unobtainable item boxes located under the S curve of DK Summit.

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. These were all changed in later appearances the track had.

In the Mario Kart Wii course "Moonview Highway", billboards can be seen for "The Mushroom Moon", featuring Petey Piranha climbing a building. This may be a reference to the movie poster for the 1933 film "King Kong". In Mario Kart Tour, this poster is updated to replace the 3D renders with 2D artwork, and replaces Petey Piranha with Bowser.

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, 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.