By datamining a demo build of the game, an image of Donkey Kong Jr. can be found, suggesting that he may have been planned to appear in the game instead of Diddy Kong. Interestingly, Diddy Kong's internal filename in the final version is listed as "dk_jr," suggesting that Diddy's appearance may have been a later addition.
The game contains two unused hot air balloon models with the "Mario Kart" title written on them. Despite the fact that they cannot be found in the final game, both can clearly been seen in pre-release screenshots of the "Dry Dry Desert" level.
In the E3 2003 version of the game, the logo for Luigi Circuit was different.
In early screenshots of Mario Kart: Double Dash!!, Luigi Circuit has larger mountains, more sunflowers, is missing the "Luigi Circuit" sign, and no arrows painted on the road.
Rainbow Road contains a heart placed outside the skybox, which uses the red shell texture on the front and a yellow texture on the back.
A lot of the tracks in Double Dash are connected by landmarks in the background. For instance, it's possible to see the mountain from D.K Mountain in the background of Dino Dino Jungle.
Entering L, R, L, R, X, Y, X, Y, Z from the menu shown after a race will reveal a code on the bottom of the menu. Despite being present in all versions of the game, this code has only been used for a contest in Japan.
By holding X or Y when the Nintendo screen is shown and 'Nintendo' is spoken, Lakitu will appear with a fish on the end of its fishing pole instead of the usual traffic lights.
The similar structure of buildings in the game's stage preview icon suggests that Rainbow Road takes place above Mushroom City, another course in the game.
There's an error on both the PAL and US box art of Mario Kart: Double Dash which displays the "L" on Luigi's cap as being backwards.
Luigi's Mansion can be seen in the background of Luigi Circuit. It's the same mansion seen during the Rank A ending of the game Luigi's Mansion.