Continuity mistake: When the book "Roots of Coincidence" is first seen it's a paperback but later in the episode it's suddenly a hardback.
Revealing mistake: When they change Professor Edgington from ape form to human form, the human form is obviously a lifeless mannequin. (00:23:35)
Answer: The cello is a large four-stringed instrument, which, when it is played, stands vertically on the floor between the player's legs (assuming they are seated). If it is to grow as large as the other instuments mentioned will, it would require a rather unseemly lack of femininity to be able to encompass it with the legs.
Rooster of Doom