Audio problem: When Kryten starts looking over Lister's bunk trying to find the Polymorph, posing as the boxers, the boxers, and therefore the Polymorph, are visible on the bunk. The sound of the Polymorph transforming is heard, indicating the Polymorph has changed form, but the boxers remain there, without any transformation. It is not until a few shots after that the Polymorph has appeared to have changed form. (00:10:00)
Timeslides - S3-E5
Audio problem: In the photo of 'Smeg and the Heads' you can see that the drummer frantically plays the cymbals, but no sound comes from them. (00:11:40)
The Last Day - S3-E6
Audio problem: When Lister tries to shoot Hudzen, Hudzen grabs Lister and throws him, but a punch sound effect can be heard, despite no punch being made. (00:26:15)
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