In Super Mario 64 DS, normally only Mario can fight King Boo to obtain the key to unlock Luigi. When Mario enters the boss battle room, he sees that his reflection in a large mirror changes to look like Luigi. Internally, this is accomplished by mirroring the movement of the control points ("bones") of Mario's model and simply applying the bones to a Luigi model instead. Since the two are so similar in build, the result appears natural.
However, if glitches are used to bring Wario into the boss battle instead, it becomes apparent that this method was not designed to work with anyone except Mario. Since Wario's bones are arranged very differently from Mario's, Luigi's model becomes distorted and unnatural upon copying them, as seen in the screenshot.
Note that the screenshot was taken by increasing the internal resolution of the game and this could not be seen in such clarity on original hardware.
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