Plot hole: Lister is not allowed any dangerous objects that he might try to commit suicide with, yet when he is with the canaries on their missions, he carries an assault rifle. The canaries aren't always supervised and it would be impossible for a guard to stop any of them putting a gun to their head and pulling the trigger. (00:05:05)
Continuity mistake: In the scene on the Canaries' transport, where they first try to convince Kryten to record the women showering, Kochanski speaks the first line in the scene, but when the shot changes to the wide shot, she isn't there anymore. (Confirmed on the DVD commentary). (00:05:55 - 00:07:20)
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