Just watching Suitable for Framing and I noticed the Saint-Saens Aquarium similarity also. Definitely intentional as just after the descending sequence of similar notes there are two alternating notes which are also in the Aquarium piece.

Interesting thread - thanks for starting it.

I don't see the similarity between the Chopin Etude and the Aquarium in "Carnival of the Animals" though. For one thing the Etude is a solo piano piece and the Aquarium features strings and a flute - and the melodies are completely different as well!

Well, the beginning of the Etude is quite different, but listen to the dark virtuoso middle part of it!!! The similarity is striking.

During the funeral scene of A Friend in Deed, we can hear an organ transcription of this tune:

BACH: "Jesu, Joy of Man's Desiring", from Cantata No. 147 BWV 147 "Herz und Mund und Tat und Leben".

For those who wonder, the original title of this Choral is 'Jesu bleibet meine Freude', and an English translation of the cantata name could be 'Heart and Mouth and Deed and Life'.

During the scene at the Italian dentist's, we can hear this opera tune:

VERDI: La donna è mobile, from the Third Act of Rigoletto.

In Candidate for a Crime, what music is playing when Jackie Cooper’s wife (Joanne Linville) is sitting on the couch calling for Juanita when her drink is empty right before her surprise party? She gets up and turns it off. It was an instrumental.

It's library music.
Anonymous