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.