Revealing mistake: The metal shelf the three spare heads sit on can be seen to move as spare head three speaks. (00:19:40)
Revealing mistake: When it is revealed that the DNA modifier has made Lister smaller, Rimmer, Cat and Kryten all have shadows cast on the floor, but Lister does not. (00:25:25)
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