Continuity mistake: When Lister drags Kryten through a doorway by his groinal attachment, he is using one hand. In the next shot he is using both hands. (00:12:05)
Continuity mistake: Just after Kryten says 'you bet your ass' he is holding onto his groinal attachment over his shoulder. In the next shot, his arm is lowered. (00:12:00)
Continuity mistake: When the gunman ties the cable to himself, he walks past a window, then in the next shot he is stood in the centre of the same window. (00:15:40)
Continuity mistake: When Lister pulls Kryten through a doorway by his groinal attachment, Kryten's cigarette is in his left hand. When the shot changes, the cigarette is now in Kryten's right hand. (00:12:05)
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