Easter egg

An Easter egg is a minor secret in a game that was hidden by the developers and are usually not associated with the gameplay.

Donkey Kong

 * In the Atari 800 version, losing Mario's last life because of a fall, while in a point range of 33,000-33,900 (thus ensuring a new high score), and highlighting a Fireball in the difficulty selection will cause the initials of the port's developer, Landon Dyer, to appear in the title screen while the player waits for the attract mode to start up again.

Donkey Kong Country (Game Boy Color)

 * The "Rareware Cow" can be found in the level Slipslide Ride.

Donkey Kong Country Returns

 * In Button Bash, there are references to Billy Mitchell (a statue with his tie) and to the Three Wise Monkeys.
 * In Foggy Fumes, the level 25m from the original Donkey Kong arcade game, Mr. Game & Watch, and a crane resembling Crocomire's skull from Super Metroid can be seen in the background.
 * In the options screen, a coral reef is shaped like the Rareware symbol.
 * In Wonky Waterway, the last part shows an 8-bit Donkey Kong statue holding a Wii Remote.
 * During Donkey Kong's idle animation, he pulls out a Nintendo DS and can be heard playing Donkey Kong.

Donkey Kong Country: Tropical Freeze

 * As Retro Studios had previously developed the Metroid Prime series, two Metroid Easter eggs can be found in the game:
 * Samus' Gunship can be found in the background in Busted Bayou.
 * A Metroid can be found in the background in Amiss Abyss, if the player passes the first signpost and giant illuminating statue, reaches the end of the trench and heads back towards the statue through the anemone. (The player must have more than one hit point to do this.)
 * During Donkey Kong's idle animations he pulls out a Nintendo 3DS and can be heard playing one of several games, including Mario Kart 7, Donkey Kong Country Returns 3D, Animal Crossing: New Leaf, and Super Mario 3D Land.
 * In the Nintendo Switch port, Donkey Kong's idle animations have him pull out a Nintendo Switch in Handheld Mode, playing the Switch port itself.

Super Mario RPG: Legend of the Seven Stars

 * In Super Mario RPG: Legend of the Seven Stars, there is a room in Booster Tower which features two curtains and a pillar, which will transform Mario into his Super Mario Bros. appearance. Mario will revert back to normal after leaving the room or after a few minutes pass. This can only be done once per game.

Paper Mario

 * There is a room inside Boo's Mansion containing a vase that will transform Mario when jumps inside into 8-bit Mario and Mario's theme will play. Mario will revert to back to his standard form if he leaves the room or jumps back into the vase. Unlike Super Mario RPG in which the 8-bit trick can only be performed once, in Paper Mario it can be performed repeatedly.
 * Similarly, Mario's theme can be heard if the player waits on the chapter screen long enough.

Paper Mario: The Thousand-Year Door

 * During Princess Peach's Interlude after Chapter 5, TEC-XX asks Peach to drink an invisibility potion in order to secretly enter Sir Grodus' room and upload important data on a disk into the room's computer. When this happens, the computer screen shows the Famicom Disk System boot-up sequence which is then shortly followed up by an animation and some music from an unknown game.
 * Similar to the first Paper Mario, in the X-Naut Fortress there is a room that, when entered from above into a changing room, makes both Mario and his partner an 8-bit sprite. This wears off after leaving the room or after a period of time. This can be done repeatedly.

Paper Mario: Color Splash

 * When using the Megaphone Card, if the player shouts into the microphone on the Wii U GamePad, it would be heard in the game, resulting in an "Excellent" Action Command and blowing enemies off-screen.

Mario & Luigi: Superstar Saga

 * If the game is played on a Game Boy Player and a second controller is plugged into the GameCube, it functions similarly to a makeshift co-op mode.

Mario Kart: Double Dash!!

 * Luigi's NEW Mansion (specifically the Rank A mansion) from the ending of Luigi's Mansion can be seen in the background of Luigi Circuit.

Super Mario 64

 * When jumping in shallow water, a fish will rarely jump out alongside Mario.
 * At the end of Bowser in the Sky, images of 8-bit Bowser and Mario from Super Mario Bros. can be seen on the pillars.
 * When entering the mansion door in Big Boo's Haunt, rarely, a low-pitched Boo laugh can be heard.

Super Mario Sunshine

 * In Noki Bay's third mission, Red Coins in a Bottle, the building at the very bottom houses a book behind an unmovable door.
 * The island of Isle Delfino itself is shaped like a dolphin, an allusion to the code name of the GameCube while it was in development.

Super Mario Galaxy

 * In the Rolling Gizmo Galaxy, if Mario or Luigi skips the Star Ball and jumps up the blocks and raised bridge that the player would normally smash with the ball, a Rupee from The Legend of Zelda series made out of blue Star Bits can be seen if the player looks down.
 * In the Toy Time Galaxy, on the planet with the first Spring Mushroom, if the player goes on the edge of the block below the warp pipe, they will notice that there is an opening. If they then go to first person view mode once in there, they will see a toy train.

Super Mario Galaxy 2

 * Three alien-like figures appear in the background of Shiverburn Galaxy watching Mario/Luigi (It also appears in the Pull Star planet of Grandmaster Galaxy).
 * In the Flash Black Galaxy, the silhouette of a windmill can be seen at the very back of the Maze Planet.
 * On the Starting Planet of Sky Station Galaxy, the sign in front of the door is usually signed "Yoshi". However, during the Prankster Comet mission, it is signed "Yoshi the space dragon".

Super Mario 3D Land

 * Using the binoculars in certain levels will allow the player to spot a passing UFO in the sky.
 * In World 4-4, when the player reaches the end of the level and stands at the gates to the right for a number of seconds, a ghostly figure will appear slowly out of the background. Once enough time passes, it will open its mouth wide and flash, while fading out at the same time. The figure can also be found at the beginning of World 8-4 and Special 6-5 if the player stands by the window long enough.

New Super Luigi U

 * In every level in the game, there is a hidden image of Luigi. Every level has one hidden Luigi except for Flying Squirrel Ovation, which has two.

Super Mario 3D World

 * Most of the levels and some of the maps feature an 8-bit Luigi, similar to New Super Luigi U, though they use the original Super Mario Bros. sprite instead of the modernized sprite.
 * In The Great Tower of Bowser Land, when interacting with the smoldering remains of the Koopa Chase Lv2, sound effects from Super Mario World will play on occasion.
 * In the credit sequence when it fades out to the sky, the Comet Observatory is visible.

Super Mario Maker

 * If the player places a pair of door objects and taps on one, knocking can be heard on the other side of the door. If the player taps enough times, Weird Mario will appear out of the door in a random powered-up state depending on the game style (for example, he can be Raccoon Mario, Tanooki Mario, Statue Mario, Frog Mario, or Hammer Mario when this is done in the Super Mario Bros. 3 style).
 * In the Super Mario Bros. style, if the fortress behind the Goal Pole has a locked door on its entrance and the player has at least one key, when they touch the flagpole and enter the fortress, the door will open as if the player unlocked it in-game.
 * Similarly, the same thing happens with a normal Warp Door or an active P Warp Door if it is placed in front of the entrance.
 * If the player shakes a Spike Trap in the New Super Mario Bros. U ground theme, a train whistle can be heard.
 * Holding, , and while a course loads will activate a CRT screen filter, similarly to the Big Mushroom; unlike the Big Mushroom, this is functional in all four game styles.

Super Mario Odyssey

 * If there are no controller inputs for a long time, a little bird will sleep on Mario's nose. The type of bird changes depending on what kingdom Mario is in.
 * Similarly, if Mario sleeps in a place where the Nintendog appears, it will sleep next to him.
 * Bowser's reaction quote to Mario changes according to the costume Mario wears. As of the version 1.2.0 update, Luigi also gives a reaction quote.
 * Like Super Mario 3D World, 8-bit character sprites can be found in several levels.

Super Smash Bros. Brawl

 * In Super Smash Bros. Brawl, a picture of a cat appears in a cabin in Pokémon Stadium 2 when the stage is Ice-themed.
 * This cameo returns in Super Smash Bros. for Wii U, but with a different cat picture.

Captain Toad: Treasure Tracker

 * Like Super Mario 3D World, 8-bit sprites of that game's playable characters can be found in several levels.

Wario World

 * If the player remains on the pause screen for 50 minutes, Wario apologizes for his taunting and stops; this applies every time the player pauses.

WarioWare: D.I.Y.

 * If a record, comic, or microgame is named "Mario Paint," a remix of the "Creative Exercise" song from Mario Paint plays while it is created.
 * Similarly, when stamps are selected, a remix of the "Monkeys" song from the same game plays.
 * This also applies to stamp editing, where a remix of the "Mysterious" song also plays.