Emohawk - Polymorph II - S6-E4
Revealing mistake: When the grenade explodes, the explosion is to the left of where the grenade landed. (00:26:50)
Revealing mistake: When the simulant pulls the knife out of his chest, you can see the knife is a dummy. As the handle is pulled back, the blade does not move at all. (00:25:30)
Revealing mistake: The low Cat's teeth move and wobble in ways they should not be able to when he is first seen. (00:14:10)
Confidence and Paranoia - S1-E5
Revealing mistake: Rimmer asks Confidence and Lister "Where's Paranoia?", when Confidence replies "I dont know. Is it some place near Uruguay?" Lister is mouthing his lines. (00:24:50)
Revealing mistake: In several episodes, but mostly in series 6, we see lots of profile camera shots of just Lister and The Cat at the front of the cockpit in Starbug. Quite a few times you will see Lister mouth The Cat's lines so he knows when to start speaking and what to say.
Revealing mistake: After the Cat's dance, Lister says 'we are outta here' and pulls the level, at which point the crew all fall back as if Blue Midget had rushed forwards. However, in the model shot of the ship it shows it scraping its foot on the floor before it runs forwards, meaning the crew should not have fallen back in their seats. (00:10:50)
Confidence and Paranoia - S1-E5
Revealing mistake: In the captain's office, the words 'Medical bay' have been painted on the wall. This was near The End of the episode and can be seen over Rimmer's shoulder while they are talking in the drive room. The set was obviously doubled with the medical bay, as the captain's office is not used in this episode. (00:23:25)
Revealing mistake: Just after we see the first meteorite, there is a brief moment where the model shot and the crew walking into the cockpit overlap causing both images to be seen together. (00:16:55)
Revealing mistake: During the fight, there is a man in a 'Flash Gordon' t-shirt with the logo forwards. (00:24:45)
Revealing mistake: When the Cat is pushing the trolly of cigarettes, Rimmer walks past him and bangs his arm against it. (00:09:30)
Revealing mistake: Just before the Cat starts his dance, Kochanski is at the back of the cockpit of Blue Midget and is bouncing up and down, yet the ship is not moving. When it starts moving, Kochanski is sitting still. (00:08:05)
Revealing mistake: When Holly gives Rimmer a new uniform, during the switch, Holly can be seen to jolt on his screen in the background. (00:03:20)
Revealing mistake: The pyrotechnic charges are visible under the 'High' Cat and Kryten's robes, before they are shot. There is also a wire trailing behind Kryten and running up his left leg, presumably a means of detonating the charges. (00:12:45)
Rimmerworld - S6-E5
Revealing mistake: When the Cat says 'Form an orderly queue behind the gunsight' you can see Lister mouth the line. (00:15:40)
Revealing mistake: When Lister slams his nose into the door of the cabinet, you can see it is nowhere near it. (00:17:50)
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)
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