Revealing mistake: When Legion gets hit, you can see his real face for a second when he adjusts his mask. (00:21:30)
Quarantine - S5-E4
Revealing mistake: When Lanstrom corners Lister, Kryten and the Cat, she is holding onto the fence. As she is a hologram, this would be impossible. (00:10:10)
Quarantine - S5-E4
Continuity mistake: Just before Dr. Lanstrom appears, Kryten is standing behind Lister. In the next shot, he has moved much further away from him, then moves back and forth between shots. (00:06:40)
Revealing mistake: One of the escort boots passes through a wall as it goes round a corner. (00:10:10)
Visible crew/equipment: In the medi-bay after Kochanski is shot, a white boom mic is visible in the top right corner as she tries to get up. (00:27:30)
Continuity mistake: When Lister is sent flying when the crates explode, his hat falls off, but when he sits up he is still wearing it. It then falls off again when the low Lister spits at him, yet he is still wearing it when he rolls away. (00:13:50)
Revealing mistake: When Lister goes into Rimmer's new room, there is a slight glitch with the split screen. When Lister is laughing at Rimmer's newspaper headlines on the door, the camera changes and the first frame only has Lister and Rimmer with the scutters. In the next frame the second Rimmer appears and the scutter on the right has moved. (00:06:10)
The Last Day - S3-E6
Continuity mistake: When Kryten takes the invitation from the skutter, he picks it up with his right hand. In the next shot it has jumped to his left hand. (00:10:20)
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: At the start of the episode when young Rimmer is hanging from the tree, his arms are by his sides. In the next shot his left arm is bent at the elbow. (00:01:00)
Revealing mistake: When Ace accelerates away from the Nazi troops, he is holding a gun in his right hand. He should not have been able to accelerate away as the throttle handle is on the right. (00:03:45)
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