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It has been requested that this article should be rewritten and expanded to include more information.
Pre-release and unused content are components of a video game that, for whatever reason, are removed or altered before that game is released. Known info about pre-release elements usually comes from one of two sources. The first is information released about a game by its creators before the game is complete. The second is data found within the coding of a game that serves no purpose but could have been implemented into the game at one point.
A room in an early version of Super Mario World
's castle (top), and the same room as it appears in the final game (bottom).
Such content get their name from the period at the end of a game's development known as Beta Testing, when the nearly complete build of the game, the beta build, is debugged and playtested by a small group of people. Despite this, games are complete or nearly complete when beta testing takes place, hence most pre-release content are scrapped long before this period. However, in case these elements are still there, they get changed based on the testers' experience. The optimal change according to the developers remains in the final game.
Pre-release elements should not be confused with vaporware, entire games which never see release.
The full list of pre-release elements can be found here.