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Missingno
From Unmariowiki
missingno is a glitch. He appears as a boss in Super Mario And Luigi: The Untold Story. To defeat him, the player must play as only luigi and say WAKLSHGSDKFGH into the mic. 45454353535235252535353552345345 times. After a while, missingno. will appear and say to STFU. The end.