Continuity mistake: When Lister sarcastically thanks the Cat for visiting him, Lister looks forwards, but in the next shot is looking at the Cat. (00:03:35)
Revealing mistake: One of the escort boots passes through a wall as it goes round a corner. (00:10:10)
Visible crew/equipment: When the crew are standing before the Judgement computer (when Lister is worrying that he will be sent to prison) if you look above Lister's head you can see a microphone bob up and down into shot. (00:11:10)
Factual error: The justice computer states that Rimmer is guilty of 1167 counts of 2nd degree murder, each carrying a sentence of 8 years giving a total of 9328 years. This is equivalent to 1166 counts. (00:14:00)
Revealing mistake: When the simulant pulls the knife out of his chest, you can see the knife is a dummy. As the handle is pulled back, the blade does not move at all. (00:25:30)
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