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A glitch or bug is an unintended behavior of any video game that results from programming errors. They range from characters falling through the floor to the game crashing. An example of a well-known glitch is the Minus World from Super Mario Bros.
When glitches occur, 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 will appear. Some glitches are caused by damaged game media, even if there are no errors in the game's programming, or other direct interference with the game media, such as cartridge tilting in Super Mario 64. Some only activate when the player presses a certain sequence of button commands. 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 glitches grant access to slapdash areas (sometimes composed of many garbled symbols) and unused parts of stages.
One type of programming error that can result in glitches across multiple games is an integer overflow or underflow, where an integer variable, instead of being stopped from increasing from its maximum value or decreasing from its minimum value, instead rolls over from its maximum to minimum or minimum to maximum value. Instances of an integer overflow or underflow causing a glitch are present in Paper Mario and Super Mario 64.