Revealing mistake: The first time you see Lister, the Cat, Kochanski and Kryten in the AR machine you can see the cat opening his eyes realising that it is not in the script, smiling and then closing his eyes. (00:04:50)
Revealing mistake: After the Cat's dance, Lister says 'we are outta here' and pulls the level, at which point the crew all fall back as if Blue Midget had rushed forwards. However, in the model shot of the ship it shows it scraping its foot on the floor before it runs forwards, meaning the crew should not have fallen back in their seats. (00:10:50)
Revealing mistake: Just before the Cat starts his dance, Kochanski is at the back of the cockpit of Blue Midget and is bouncing up and down, yet the ship is not moving. When it starts moving, Kochanski is sitting still. (00:08:05)
Revealing mistake: When Rimmer is typing on the keyboard, what he is saying does not match what he is typing. He is typing randomly. (00:12:20)
Revealing mistake: When Rimmer puts the engaged blindfold on and sits on the toilet, he doesn't take his trousers down. (00:03:50)
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