Gunmen of the Apocalypse - S6-E3
Revealing mistake: When the crew are trying to figure out how to save Kryten before going into the game, the Cat says 'if not, why not?' You can see Lister mouth the line. (00:15:25)
Gunmen of the Apocalypse - S6-E3
Revealing mistake: When Rimmer throws a cowboy over the bar, it wobbles. (00:21:35)
Gunmen of the Apocalypse - S6-E3
Revealing mistake: When the Cat says 'not if you believe those lying cartoons' you can see Lister mouth the line. (00:09:50)
Gunmen of the Apocalypse - S6-E3
Revealing mistake: At the start of the episode, we see Lister in black and white talking to Loretta. Then we cut back to what's really happening, he is playing on the AR console. Look at the door behind him, you can see Kryten's hand sticking out in the doorway. He is supposed to rush in and start talking, but you can see him waiting off camera to be told when to enter the room.
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