Glitch

"Glitch" is the term applied to something strange happening in a Mario game, as well as any other video game. They are caused by programming errors within the game's coding. Arguably, the most famous (or infamous) glitch in video gaming history is the Minus World glitch from Super Mario Bros.

When glitches happen, either something unusual will happen (such as Mario being able to walk underwater, a glitch in Super Mario Sunshine and Super Mario 64 DS) or the screen will freeze and sometimes some random pixels and object tiles appear on screen. Some glitches are caused by damaged games. Some only activate when the player presses a certain sequence of input commands (pressing buttons). Also, as seen in Super Mario 64, if a player is to tilt the game console itself, Mario's limbs will be dislocated and he will sink into the ground, although players can still play in that position. However, entering a door like that will freeze the game. Other glitches, such as the aforementioned Minus World, can actually create new levels occasionally made randomly and haphazardly, and others allow players to guide their character off-screen, sometimes granting them access to other glitch-like areas (usually composed of many garbled symbols) and even unused parts of stages.

The full list of glitches can be found here.

Trivia

 * Glitches have been parodied and even used in the plot of Mario games. Examples of this include TEC-XX, who had the "glitch" of falling in love with Princess Peach in Paper Mario: The Thousand-Year Door, and Fracktail of Super Paper Mario, who shouts many lines of dialogue spoofing glitches and errors.