Easter egg

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An Easter egg in Super Mario 64

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.

Easter eggs in Mario games

Donkey Kong series

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.

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 series

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.

Super Paper Mario

In Flopside, if Peach talks to Bleu, he will change Peach's hairdress. If she gets into an elevator or if the player switches characters, Peach will get back her usual hairdress.

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 series

Super Mario Galaxy

In the Rolling Gizmo Galaxy, if Mario or Luigi skips the Star Ball and jumps up the blocks that if the player would smash with the ball, and then up the raised bridge, if the camera is pointed down a Rupee from The Legend of Zelda series made out of Star Bits.

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 hell valley sky tree will appear slowly out of the background. Once enough time passes, it will unexpectedly open its mouth wide and flash, while fading out at the same time.

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.

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.

Captain Toad: Treasure Tracker

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