Revealing mistake: When Lister goes into Rimmer's new room, there is a slight glitch with the split screen. When Lister is laughing at Rimmer's newspaper headlines on the door, the camera changes and the first frame only has Lister and Rimmer with the scutters. In the next frame the second Rimmer appears and the scutter on the right has moved. (00:06:10)
Revealing mistake: When the two Rimmers are exercising, there are no stars visible out of their window. (00:11:20)
Revealing mistake: When Lister goes to Rimmer's room the second time, the door opens before Lister hits the switch.
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