Revealing mistake: After the T-Rex eats the curry, it starts stamping about the cargo bay, yet there are no ripples in the very watery curry. (00:08:30)
Revealing mistake: When Rimmer and Lister are being forced out Captain Hollister's office due to the effects of the fight catching up with them, a rattling sound can be heard off-screen, indicating the use of a hidden trolley-like device pushing them into the wall. (00:21:55)
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