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)
Revealing mistake: When Lister and Rimmer first meet their femaile opposites, Lister takes a step back and brushes against Rimmer's hologramatic shirt. (00:10:30)
Continuity mistake: At the disco, when the Rimmers are seated at their table, the plant moves from in front of the lamp to behind it. (00:15:40)
Continuity mistake: In the disco when Lister complains about the female Lister's attempts to impress him, his hair is in front of his left shoulder, then behind it. (00:19:10)
Continuity mistake: When the Cat jumps up to grab the mirrorball, a comb appears in his hand from nowhere before disappearing again. (00:20:25)
Continuity mistake: When the Cat dances at the disco, he jumps up and grabs the mirror ball with both hands. In the next shot he is only holding it by one arm. (00:20:25)
Continuity mistake: When Lister and Rimmer talk to Holly at the disco, Lister's arm is on the table. When the camera angle changes, it is on his lap below the table. (00:21:15)
Continuity mistake: In the quarters of the female versions of the crew, the newspaper clippings and certificates on the wall still have the name 'Arnold' on them, and not the female equivalent, as it was supposed to be. (00:23:15)
Visible crew/equipment: A wire/string pulling along the baby Skutters behind the parent Skutter is visible in the episode. (00:25:35)
Revealing mistake: During the last scene of the episode, the Cat keeps brushing against Rimmer's hologramatic shirt. (00:27:10)
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