Helen Keller! The Musical - S4-E13
Corrected entry: After Cartman's mom tells him he has twenty minutes to finish his play, Cartman says "Did Tim Rice's mom give him twenty minutes to write the lyrics to 'Phantom of the Opera?'" Tim Rice did not write the lyrics to "Phantom of the Opera," Charles Hart did. This was not a deliberate mistake or a comment derived from Cartman's stupidity, as musical theatre is too obscure of a topic for fans to laugh at when Cartman screws up its information.
Correction: As a matter of fact, it is precisely a character's mistake due to Cartman's obtuseness, and it is not that obscure of a topic if a fan just like yourself picks up on his blunder.