Confidence and Paranoia - S1-E5
Revealing mistake: After Lister faints, you can see him smirk when the cat sings 'S-E-X, I think I've found it!' (00:08:50)
Revealing mistake: The first time you see Lister, the Cat, Kochanski and Kryten in the AR machine you can see the cat opening his eyes realising that it is not in the script, smiling and then closing his eyes. (00:04:50)
Revealing mistake: When the cat priest dies on the bed, watch him move his foot so Lister can sit on the edge. (00:26:40)
Revealing mistake: When Kryten is doing his driving test, as soon as we see Rimmer and Kryten in the cockpit you can see the wire attached to Chris Barrie behind him, next to the aerial on his hat, going vertically out the top of the frame. (00:03:40)
Revealing mistake: Just after the Trojans' conversation, there are two jolts. Lister jumps back too soon on the second jolt. (00:03:30)
Out Of Time - S6-E6
Revealing mistake: When the crew wake up from stasis, there is a model shot of Starbug flying towards the epicentre. You can see the legs of Starbug partially disappear as it gets closer to the ship. (00:12:45)
Confidence and Paranoia - S1-E5
Revealing mistake: When Rimmer gets into bed, near the start of the episode, you can see that he brushes past Lister's blanket hanging down and slightly moves it. He should not have been able to do this as he is a hologram, it should have passed through him. Rimmer's pillows and bedding move as well. (00:06:50)
Revealing mistake: When Lister leaves for his exam, he walks through Rimmer. As he does, both Rimmer and Lister become transparent when it should have just been Rimmer. (00:25:50)
Revealing mistake: The cigarette butts that Lister supposedly tips out of his lager can were obviously in between his hand and the can, and not in the can at all. (00:19:25)
Revealing mistake: During the 'Two Listers' scene, specifically the cut when Kryten tosses the apples to the Listers, the split screen is quite visibly moving in between the chairs. (00:22:55)
Revealing mistake: Starbug and Blue Midget wobble on a number of occasions during the chase. (00:23:25)
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