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 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 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.
 * In Donkey Kong 64, a picture of a strange frog appears in Angry Aztec.
 * In 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 could only be performed once, in Paper Mario it could be performed repeatedly.
 * 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.
 * 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. 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 ghost figure 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.