Revealing mistake: When Rimmer runs to Lister after Lister unsuccessfully orders a black coffee, the camera tilts.
Revealing mistake: When the future echo Rimmer leaves the drive room and the actual Rimmer walks in, there is a visible line caused by the split screen effect, done to show two Rimmers onscreen at the same time.
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