Continuity mistake: When Kochanski is banging on the pipes at the start of the episode, she is using her right hand. There is then a close up shot of the pipe and you can see she is using her left hand. (00:01:50)
Visible crew/equipment: When Lister sneaks in to switch Kryten's head, as the camera follows Lister and goes around past the wall, the shadow of the camera can be seen in the green light. (00:09:20)
Continuity mistake: When Lister pulls Kryten through a doorway by his groinal attachment, Kryten's cigarette is in his left hand. When the shot changes, the cigarette is now in Kryten's right hand. (00:12:05)
Continuity mistake: When Kryten says 'you bet your ass' his cigarette has switched hands between shots. (00:12:00)
Continuity mistake: When Able screws Kryten's head on, he is standing in front of him. In the next shot, Able is behind Kryten next to the table. (00:15:30)
Visible crew/equipment: When the crocodile is thrown at Ace, the crash mat is visible as he lands. The mat disappears in later shots. (00:01:00)
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)
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)
Revealing mistake: When Able screws on Kryten's head he only turns it once, yet in Tikka To Ride, Lister screws his head on at least twice around. (00:15:30)
Continuity mistake: After the Cat says about mooning out of the windows when Starbug gets hit, Kochanski falls back on her chair with her arms outstretched. In the next shot she is holding onto the monitor again. (00:25:00)
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