Continuity mistake: When the Cat says, "So what do we do with them?" Kryten disappears from Lister's left between camera shots. (00:17:30)
Revealing mistake: When Rimmer wakes Lister up, as he stands up you can see he moves Lister's duvet with his shoulder, which he can't do, being a hologram. (00:15:20)
Visible crew/equipment: In the cell at the start of the episode, after Rimmer says they have to escape, a boom mic is visible at the top of the screen. (00:03:10)
Continuity mistake: When the simulant walks past Lister, the Cat and Kochanski who are hiding under the blanket, he walks to the far right of the shot and a piece of material is hanging from him as he stops. The shot changes to show the simulant walking through the doorway, yet when the camera cuts back to Lister, the Cat and Kochanski, the piece of material is still in the corner of the screen meaning the simulant is still stood there. (00:11:20)
Continuity mistake: When Ace is firing at the Nazi guards from his parachute, a white sleeve is visible. In all other shots his sleeves are brown. (00:03:00)
Rimmerworld - S6-E5
Visible crew/equipment: When Rimmer argues that they should not board the simulant ship, the shot of Lister in the foreground and Rimmer in the background shows a boom mic just above Rimmer's head. (00:04:40)
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 does his tap dance, he starts 20 feet away from Blue Midget and does not move from his position during the dance. When he finishes, he taps Blue Midget's leg. (00:09:30)
Quarantine - S5-E4
Continuity mistake: When Rimmer is looking at the crew through the viewscreen in Quarantine, the reflection of the crew in the glass changes between shots. (00:21:20)
Continuity mistake: When Starbug jolts for the first time, Rimmer is looking to his right. The camera cuts and he is looking forwards. (00:03:30)
Plot hole: Allman does not remember his trial when The Inquisitor erases him, yet Lister and Kryten do remember their trial when they are sentenced. (00:01:20 - 00:13:30)
Continuity mistake: At the start of the episode when we first see the polymorph, it looks into the mirror, then covers its eyes. Yet when we see the reflection, it already has its hands over its eyes before it gets to the mirror. (00:01:30)
Stasis Leak - S2-E4
Continuity mistake: At The End of the episode when Lister is talking to Holly on his watch, Lister's red sleeve is in line with the watch, but as the camera cuts to show Holly, the sleeve is gone. (00:24:10)
Revealing mistake: When Holly shows the crew the Holly Hop Drive, you can see the outline of his black clothes throughout the scene. (00:06:35)
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