Rayman - Did You Know Gaming? Feat. Caddicarus
In the Nintendo 64 version of the game, in the cutscene during the Iron Mountains level where Rayman is riding the hot air balloon, repeatedly mashing the Z button before Rayman jumps off will cause one of two different hidden messages to appear:
These are both references to Duke Nukem.
Contributed by MehDeletingLater
Though the 3DS port’s North American boxart is nearly identical to the original Nintendo 64 box art, the PAL boxart instead features Rayman holding a Yellow Lum and smiling to the camera. This boxart is also mirrored in the German release, most likely to prevent the rating label from obscuring Raymans face.
Contributed by CuriousUserX90
An iOS version of Rayman 2 was once available on the App Store, published by Gameloft, and developed by Ubisoft. It was directly based on the Dreamcast version of the game and included all of the changes made to that version. However, all minigames and bonus games were removed, most likely due to file size restrictions. It was later removed from the App Store 3 years after its release for unknown reasons.
Contributed by The_Maxious
In early copies of the PC version, the game would sometimes mistakenly display the "CD missing!" message. The game is still fully playable, however, with the drawback of the error message obstructing most of the screen.
Contributed by game4brains
In the final phase of the Tomb of the Ancients (outside the door to the Clark boss fight), there is a secret passage leading to a blocked-off area showing a campfire party with a dancing Grand Minimus, two baby Globoxes playing drums, and a large character named Jeff, a depiction of Ubisoft lead 3D programmer François Queinnec, playing guitar.

This secret was originally created by an unknown artist and developer Arnaud Guyon for the Nintendo 64 version of the game and can be accessed with the following combination: Hold L + R while pressing A/B/A/B/A/B. This will cause a crate to appear that can be used to enter the passage. When the PC version of the game was developed, this cheat was changed to standing in a specific spot in front of the passage and typing "playjeff" to make the crate appear. Guyon posted details of this variant of the Easter egg on his website in 2002, but it would go undiscovered by players until 2018.

This area can also be accessed in the Dreamcast and Nintendo 3DS versions of the game using the following combinations:
Dreamcast: Press A/B/X/Y/X/Y
Nintendo 3DS: Press B/Y/D-Pad Right/D-Pad Left/D-Pad Right/D-Pad Left

In the Nintendo DS, iOS and PlayStation 2 versions of the game, the hidden area is available, but can't be accessed, either due to the respective buttons not being available to press or because the cheat was disabled. In the PlayStation version, it does not appear at all.
Contributed by Yoshispacedragon
"The Crow's Nest" features a hidden collectable which gives Rayman unlimited ammunition. After gaining the ability to fire, flying to the highest part in the middle of the chamber while holding the jump and shoot buttons will result in Rayman passing through it and entering a secret tunnel. At end of the tunnel, a glowing blue ball can be collected which will give the player an unlimited number of shots.
Contributed by Yoshispacedragon
In the 3DS version, the spider-web in the Tomb of the Ancients located in front of the 1000th Lum doesn't have any bounce physics. This means that it is impossible to get the 1000th Lum without abusing a glitch in which the player must use an enemy's knock back effect to lift themselves up to it.
Contributed by Takahashi2212
In an early teaser trailer for the game, Rayman can be seeing using his fists as his attack, similar to the first Rayman game; rather than shooting energy spheres as in the final product.
Contributed by Yoshispacedragon
One of the official artworks for the game is based on the French painting "Le Radeau de la Méduse'" or "The Raft of the Medusa" by Théodore Géricault. In it, Rayman and his friends are on a raft in similar poses to those of the people in Géricault's painting.
Contributed by Blase005
In the PlayStation version of the game, collecting at least 720 Lum will unlock a secret 2D level. It is believed that this is a remaining level from the cancelled 2D sequel planned for the original Rayman.
Contributed by Tadaboody
It's possible to get a bad ending in the cave of bad dreams. After the race with Jano, select the option for riches and wealth and you will be rewarded with a cutscene of a fat Rayman siting on an island with a pile of gold next to him, with the words "The End" on the screen.
Contributed by GoldenFloor
Rayman 2 originally started out as a 2D platformer akin to the original Rayman. It was to feature the robotic pirates concept that made it into the final game, Betilla the fairy was planned to be in the game, but Ly was nowhere to be found. Many robot concepts were cut, including a robotic dinosaur.
Contributed by Liraxus
Although Razorbeard eats one of the 1,000 yellow Lums (which lowers the counter to 999), you can still find a 1,000th Lum. It can be found in the Tomb of The Ancients but it does not add to the counter - it only refills your life.
Contributed by gamemaster1991
Rayman wasn't the only one whose color was changed to the Japanese version of the game. Sssssam the watersnake went from purple to green, Jano's hat became red, and Ly the fairy's design changed completely: her eyes were rounder, her pupils were bigger and a lighter shade of green in color, she was given thin eyebrows the same color as her hair, instead of green lips, they were now pink, and her clothes also became purple and white.
Contributed by Clamanath
In the Japanese release, Rayman was recolored from purple to blue. This is because in Japan, purple is commonly used on villains. It is likely the change was to assert Rayman as the game's hero.
Contributed by DidYouKnowGaming