2D Nightmare is a extra minigame exclusively featured in the Nintendo GameCube version of the game. Its aesthetics are based on 'Picture City' from the original Rayman, and its intended unlock method is by connecting both the GameCube & Game Boy Advance version of Rayman 3, with the latter version requiring that all levels are completed. However, due to a programming error in the GBA version, the final boss level of the game 'Heart of the Ancients' is never counted as complete due to the game not saving after the credits roll. It was assumed hacking via Action Replay codes was the only way to play 2D Madness. However, 20 years after both games released, it was discovered that the GBA version contains hidden in-game cheat codes.

On the game's world map screen, holding the Select button while inputting:
• R/A/A/A Unlocks All Powers
• B/B/B/A/A/A Gives 99 Lives
• UP/LEFT/DOWN/RIGHT/B/A/L/R Completes All Levels
• DOWN/UP/DOWN/UP/A/LEFT/B/RIGHT 100% Complete of Levels, Powers, Lums, Cages & 99 Lives

The last two cheat codes mark every level in the game as complete, including the final boss level. Entering either code will allow 2D Nightmare to be unlocked officially in the GameCube version without any hacks to access.