From Method to Madness - S3-E18
Corrected entry: When Stewie is trying to play the french horn, the music score written on the wall behind him makes no sense. One line is two and a half beats long, and the next line is four beats. There are no measure lines anywhere.
Correction: This is a school for young children, it is quite possible one of them drew that music score, not knowing too much about what to do. In fact, it could have been Stewie, who's just a baby.