Audio problem: When the high Lister plays the harp, his fingers are out of sync to what is being played. (00:11:30)
Audio problem: When the high Kryten picks up the bomb, the high Cat can be seen to start speaking but no sound is heard. In the next shot, the high Cat is then smiling. (00:13:30)
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