Continuity mistake: After the Cat incapacitates Lister, the Cat complains about his collar being untidy, then steps aside. The next shot shows Lister stood on his own and the Cat is not visible at all. There was no time for the Cat to walk that fast between shots. (00:23:20)
Terrorform - S5-E3
Continuity mistake: After the crew fire at the Self-loathing Beast and the room fills with dust, the Cat's left hand is up in the air. In the next shot he is holding the bazooka with both hands. (00:17:00)
Continuity mistake: When Lister is choking the Cat, he says 'give me a break, Rimmer'; at this point Rimmer is looking at the Cat and Lister, yet in the next shot he is looking at Kryten. (00:22:50)
Continuity mistake: The bottle of chloroform Kryten uses to take Lister by surprise appears from nowhere. (00:23:30)
Continuity mistake: After the future Kryten cuts off The Inquisitor's hand with the chainsaw, he kicks the glove which lands about 7 feet away from Lister. In the next shot Lister simply bends down and picks the glove up from in front of him. (00:13:45)
Quarantine - S5-E4
Audio problem: When Dr. Lanstrom says 'Life without pain has no meaning' Kryten begins to speak but no words are heard before the shot changes. (00:07:10)
Revealing mistake: After Starbug lands on the 'high' Red Dwarf, Rimmer is running out of power and starts to fade in and out. However, Cat, who must be standing too close to Rimmer also fades, the backdrop being partially visible through him. (00:08:05)
Visible crew/equipment: When the High Rimmer and Cat dance, in the wide shot a boom mic can be seen at the top of the screen. (00:11:35)
Continuity mistake: When Lister, the Cat and their higher selves are walking down the corridor, the way Lister is facing changes between shots throughout the scene. (00:09:30)
Revealing mistake: At the very start of the episode, when the SSS Esperanto is seen, the bubbles from the seabed are sinking rather than rising, revealing the piece of footage is being played backwards. (00:00:35)
Visible crew/equipment: When Lister is being taken over by The Inquisitor, a boom mic shadow can be seen moving on Kryten's chest. (00:04:15)
Quarantine - S5-E4
Continuity mistake: When Lister gets the second luck virus injection, his locks move from in front of his shoulder to behind his back. (00:23:25)
Continuity mistake: When The Inquisitor attacks the crew in the lift, Kryten and the Cat go from crouching to standing stright up between camera angle switches. (00:18:25)
Terrorform - S5-E3
Continuity mistake: When the crew finds the charm gravestone, Kryten looks at the Cat. Then in the next shot, Kryten looks at the Cat again. (00:14:25)
Continuity mistake: When remote controlled Lister is strangling the Cat, his locks are partially over his left shoulder and partially behind his back. When Lister picks up the bazookoid, his locks are all over his left shoulder. (00:23:10)
Continuity mistake: After the Cat starts to cry, he is looking at Kryten and Lister. In the next shot he is looking down. (00:04:50)
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