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In Paper Mario: The Origami King, one of the jokes that can be optionally encountered in the brief visit to Peach's Castle late in the game is attempting to open the door on the right side of the foyer, which is locked. Bowser will then suggest there is nothing on the other side to begin with.

While this is true in the sense that there is no actual way to open the door and it does not lead to a room, looking behind the door reveals there is actually a space there that is identical to the spaces used for loading zones behind other openable doors in the game, so the door could in fact accommodate being opened and taking Mario to another room. It stands to reason that the space behind the door would not be modeled at all if it was not intended to be opened at any point, as is overwhelmingly the case with unopenable doors in 3D video games.

This suggests that the door was possibly intended to lead somewhere during development and the room was removed, and Bowser's comment is not merely making fun of unopenable doors in video games but also is inside joke for the developers. It should however be noted that it is possible that the space was simply copied over from the other side of the room when it was modeled (since the layout is symmetrical), though the room is not actually exactly symmetrical making this slightly less likely.

Source: noclip.website