Revealing mistake: When Holly shows the crew the Holly Hop Drive, you can see the outline of his black clothes throughout the scene. (00:06:35)
Revealing mistake: When Lister and Rimmer first meet their femaile opposites, Lister takes a step back and brushes against Rimmer's hologramatic shirt. (00:10:30)
Revealing mistake: During the last scene of the episode, the Cat keeps brushing against Rimmer's hologramatic shirt. (00:27:10)
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