Continuity mistake: When Lister punches up the Leviathan's destination, it reads 'Theta 4'. However, in the next scene in Starbug's medibay, Lister says that the Leviathan was headed to Delta 7. (00:23:10 - 00:24:25)
Factual error: When Kryten revives Lister with the defibrillator paddles, he places them on the clothing he is wearing. Defibrillator paddles need to be applied to bare skin for the electricity to get through. (00:27:35)
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