Bowser's Shell was originally going to be an item in Mario Kart DS, but it didn't make the cut. However, its sprite can be found within the game's files.

There are a few unused graphics stored in the game:
• An early question mark for the fake item box.
• A very basic white-colored debug font.
• A GCN Mario Circuit tree, most likely intended to appear on the unused GCN Mario Circuit track.
• A multiplayer select screen with only two cells, called select_game_m_single.
• Shy Guy character icons with nine color variations. This was likely left out because Shy Guy is exclusive to DS Download play and cannot normally be selected as a character.

In the demo, Donkey Kong was simply called "DK" while in the final game he is called "Donkey Kong".

If the game is played on an original Nintendo DS console, Mario will say "Wahoo" on the opening screen, but if played on a DS Lite console or later, Mario says "Here we go!" instead. This is due there being a random number generation system that is intended to choose two sound effects from a pool of two each, the aforementioned two Mario sounds, and two car sounds - one of a car revving up and another of a car zooming past. However, due to a fault in the programming, the number is instead determined by load time, resulting in only one sound effect each, with the original "fat DS" only having variance in Mario's voice due to it having very slightly slower hardware, and the car zooming past sound effect becoming very rare compared to the rev-up sound effect.

The kiosk demo's retro cups were named SNES, N64, GBA and GCN cup, and they only featured the courses from the corresponding consoles.

Proffesor E. Gadd was planned to be a playable character in the game, but was replaced with Dry Bones.

Even though the Chain Chomp item, which appeared in previous Mario Kart releases, was replaced with the Bullet Bill, it still appears in the item roulette animation. There is also an unused Purple Koopa Shell.

In the kiosk demo prototype version of the game, almost all of the item roulette icons were either redrawn or resized. Additionally, some of the icons were reused from Mario Kart: Double Dash!!.

A kiosk demo prototype version of the game had a blue background on the title screen, rather than a blue and white background. The logo was also changed, and there is no copyright information at the bottom.

There are nine unused tracks in the game. There is an unfinished GCN Mario Circuit, two Waluigi Pinball beta courses, an unused battle track, four test courses, and an unused Wario Stadium replica.

An unused version of Waluigi Pinball can be found in the game's code under the name "luigi_course" hinting that the track may have been originally planned to be based on Luigi rather than Waluigi.

All of the mission mode bosses within the game are from Super Mario 64 DS.

Mario Kart DS seems to pad certain parts of memory with repeated instances of ASCII art of Mario from Super Mario Bros.

ROB is the first non-Mario character to appear in the Mario Kart series. In the Japanese release, he is red and white (like the Famicom), whilst in international releases he is dark grey (like the NES). His kart is similarly recolored.

The "Airship Fortress" track is called "Killer Ship" in the Japanese release.

Shy Guy has a select icon which is unused because he is exclusive to DS Download play.

Wario has a fourth kart design but it goes unused.

It's possible Birdo was originally considered as a playable character. A pink version of Yoshi's Egg 1 kart was found in the game's data, a possible reference to Birdo's pink eggs.