Continuity mistake: The bottle of chloroform Kryten uses to take Lister by surprise appears from nowhere. (00:23:30)
Continuity mistake: Just before Kryten chloroforms Lister, Lister's locks jump from behind his back to over his left shoulder between shots. (00:23:40)
Visible crew/equipment: When the Cat shoots the locker which the low Lister is hiding at The End of the episode, a boom mic can be seen above the Cat. (00:25:40)
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