Continuity mistake: When the low Kryten takes the spinal implant from low Lister, he uses his right hand. In the next shot, the implant has switched hands as Kryten uses his right hand to lift up Lister's head. (00:16:40)
Revealing mistake: When Lister slams his nose into the door of the cabinet, you can see it is nowhere near it. (00:17:50)
Continuity mistake: When Lister crushes the high Rimmer's light bee, he puts his arm into Rimmer's chest. In the next shot, Lister's arm is gone. (00:21:35)
Continuity mistake: When Lister swings the axe at The Cat and hits the door frame, the axe head bends. When Lister pulls the axe out, its head is fine again. (00:22:30)
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: 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 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)
Continuity mistake: The bottle of chloroform Kryten uses to take Lister by surprise appears from nowhere. (00:23:30)
Continuity mistake: Just before Kryten chloroforms Lister, Lister's locks jump from behind his back to over his left shoulder between shots. (00:23:40)
Visible crew/equipment: When the Cat shoots the locker which the low Lister is hiding at The End of the episode, a boom mic can be seen above the Cat. (00:25:40)
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)
Continuity mistake: When the Cat asks if fish swim south for the winter, Kryten is looking at him. In the next shot, as Kryten answers, Kryten is looking up and away from the Cat. (00:02:05)
Continuity mistake: When the Cat is holding up the haddock, Kryten is stood in front of him holding out the scanner. In the next shot, Kryten has vanished, then reappears. (00:03:00)
Continuity mistake: When the Cat is holding up the haddock, going through the people who have commited suicide, the shot changes and the haddock disappeares from his hand. Also after the shot changes, the Cat is then pointing at Kryten and smiling. (00:03:25)
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)
Visible crew/equipment: When we first see the crew looking at the radar before the lights go out, a boom mic can be seen in the top left corner of the screen. (00:05:35)
Continuity mistake: When the crew are looking at the radar, Lister's locks are in front of his shoulder on the radar cam, but behind his back in all other shots. (00:05:50)
Visible crew/equipment: Just after Lister says the despair squid will either kill them, eat them or hump them, a boom mic shadow can be seen moving on his face. (00:06:05)
Continuity mistake: When the crew are looking at the radar, Lister is looking at the crew in one shot then at the radar in the next. (00:06:10)
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