Easter egg

An Easter egg is a small secret in games, hidden by the developers. Easter eggs usually don't associate themselves with gameplay, but players usually find Easter eggs due to interest in them.

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 64

 * A picture of a strange frog appears in Angry Aztec.

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 a 8-bit Donkey Kong statue holding a Wii Remote.
 * In Clifftop Climb, if the player looks extra hard, they will see a stone-carved portrait of King K. Rool's head.
 * During Donkey Kong's idle animation, he pulls out a Nintendo DS and can be heard playing Donkey Kong.

Donkey Kong Country: Tropical Freeze

 * Samus' Gunship can be found in the background in Busted Bayou.
 * 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.

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 8-bit Mario.

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.

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.

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 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.

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.
 * In the credit sequence when it fades out to the sky, the Comet Observatory is visible.

New Super Luigi U

 * In every level in the game there is a hidden image of Luigi.

Super Mario 3D World

 * Most of the levels and some of the overworlds in Super Mario 3D World feature an 8-bit Luigi, similar to New Super Luigi U.
 * In The Great Tower of Bowser Land, when interacting with the smoldering remains of the Bowser Mobile, sound effects from Super Mario World will play on occasion.

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.

Super Mario Maker

 * If the player places a pair of door objects and knocks on one, knocking can be heard on the other side of the door. If the player knocks 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 castle behind the flagpole has a locked door on its entrance and the player has at least one key, when they touch the flagpole and enter the castle, the door will open as the player has unlocked it.
 * If the player shakes a Spike Trap in the New Super Mario Bros. U ground theme, a train whistle can be heard.

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.

Super Mario Odyssey

 * If the Joy-Con is left alone for a long time, a bird will sleep on Mario's nose.
 * Similarly, if Mario sleeps in a place where the hatted shiba inu 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.
 * Interestingly, when wearing the Luigi costume while talking to Luigi, he'll mistake Mario for himself, similar to when playing as, and subsequently rescuing at the same time, Luigi in Super Mario Galaxy.