Other mistake: When Rimmer is doing Lister's memo, there there are only about a few stars visible out the window, which is far too few given the amount of stars shown in exterior space shots of the ship. Additionally, a star suddenly appears in the window. (00:23:15)
Other mistake: When the Cat says there better be no dogs on the ship, there are only about two or three stars visible out the window. Far too few given the amount of stars shown in exterior space shots of the ship. (00:05:15)
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