Mario Kart XXL (unreleased 2004 GBA tech demo)

Mario Kart XXL was a Game Boy Advance game being developed by Lenaris Entertainment Software. The demo was intended to show how the Game Boy Advance was capable of rendering, scaling, and rotating multiple background layers. It is unknown when this came out, but it's possible that it was made around 2003, seeing how the title screen uses artwork taken directly from Mario Kart: Double Dash. It's worth noting that this unreleased tech demo is one of the only GBA games to ever use multiple background rendering/scaling/rotating.

Word got out that this existed when a lucky YouTube user going by the name "Steven Seventyeight" found and recorded footage of the game being played on a Gamecube via the Game Boy Player accessory.

Gameplay
Mario Kart XXL has a "Demo" and "Play" mode. Demo mode plays an automatic playthrough of the tech demo that repeats, similar in vein to the Super Guides from the New Super Mario Bros. series. Play mode allows you to play through the tech demo. There is no character select; the only character available is Mario, who is pre-rendered with a custom 3D model. You can return to the main menu at any given time by pressing the X button on a Gamecube controller during gameplay.

Mario Kart XXL consists of a time trial with 3 laps. The game starts you out on a custom track reminiscent of Donut Plains from Super Mario Kart, with a second layer underneath the track depicting a beach-like area similar to Koopa Beach, also from Super Mario Kart. Coins appear on the track, but can't be collected. Mario doesn't slow down when exiting the track unlike other Mario Kart games. It's possible to fall out of bounds, respawning you on the track after hitting the bottom layer with a splashing effect, albeit without the help of a Lakitu. Transparent windows on the track that show the bottom layer of the track are present towards the beginning and end of the track. The camera zooms in and out at times, but you can control the camera manually by using the L button to zoom out and the R button to zoom in.