Easter egg

An Easter Egg is a small secret in games. Easter eggs usually don't associate themselves with gameplay, but players usually find Easter Eggs due to interest in them.

Examples of Easter Eggs in Mario games:
 * In Donkey Kong 64, a picture of a strange frog appears.
 * In the Atari 800 version of Donkey Kong, 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.
 * In Super Smash Bros. Brawl, a picture of a cat appears in a house in Pokémon Stadium 2.
 * In 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, possible to note that the Clockwork Ruins Galaxy is nearby.
 * Donkey Kong Country Returns has various easter eggs:
 * In the level Button Bash is found 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 and Mr. Game & Watch 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.
 * In Super Mario 3D Land, using the binoculars in certain levels, will allow the player to spot a passing UFO in the sky. The level World 5-2 is designed as a dungeon from the 2-D The Legend of Zelda games, even playing the "puzzle solved" tune at some point from said games.