Other mistake: Kryten places Rimmer's light bee on the floor, then begins the process of reinitialising him. As the process finishes and Rimmer looks at Lister (who looks away), the light bee, which should be hovering within his torso projecting his image, is still visible on the floor between his feet. (00:06:00)
Other mistake: When Legion stabs his left hand, everyone grabs and shakes their left hands - except Lister, who grabs and shakes his RIGHT hand. (00:22:40)
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