Continuity mistake: When Lister finishes pouring a glass of Bucks Fizz, he begins to place the jug back on the table with his hand still on it, but in the next shot, the jug is now on the table with Lister's hand nowhere near it. (00:20:20)
Continuity mistake: When Rimmer suggests bringing Legion with him, Lister says he's not going to come with them, and asks if he would ever join in on their late night sessions of pin the pointy stick on the weather girl. At The End of the line, Lister is looking to the right, but in the next shot, he is suddenly looking downwards. (00:12:35)
Revealing mistake: When Lister hits Legion over the head, his mask falls off. In the shot that shows Rimmer, Cat and Kryten in the foreground sitting at the table and Lister and Legion in the background, Legion's face can be seen and it is apparent that it is not the combination of the four that is his face, but in fact the actor's real face. (00:21:15)
Continuity mistake: When Legion puts his mask back on, just as he says "Kryten knows", the mask is not properly fixed on, leaving his chin exposed, but when the camera is on him next, the mask is suddenly fixed into place. (00:21:35)
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