Revealing mistake: When Legion places the hard light in Rimmer's light bee, it is noticeable that the hard light does not go inside the device. It is placed more to the area beside it. (00:09:55)
Revealing mistake: During the scene in the space station's dining room, when Craig Charles is making a speech explaining that they should escape, he stands up and pours himself a glass of Bucks Fizz. Craig fills the glass too soon (half way through the speach) and must realise, as he pours the drink back into the pitcher, and then begins filling his glass again. (00:20:20)
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)
Revealing mistake: When Legion gets hit, you can see his real face for a second when he adjusts his mask. (00:21:30)
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