Visible crew/equipment: When Lister steps back inside the observation room after Rimmer leaves, a stage light can be seen reflected in the window-like light fixture on the wall. It can't be a light on the ship as none of the lighting on Red Dwarf is in the shape of the light seen in the reflection. (00:14:05)
Visible crew/equipment: When Lister is putting the chicken vindaloo on the table, a stage light is visible on the left hand side of the screen. (00:11:50)
Visible crew/equipment: In the Captain's Office, when Rimmer says "That is not true. We played the joke on Mr. Ackerman, sir", a stage light is reflected on the object behind him and Lister. (00:01:05)
Visible crew/equipment: When Lister and the Cat are walking through the corridor on the ship, the top of the set is visible through the transparent ceiling. (00:03:55)
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