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: When Holly gives Rimmer a new haircut, Rimmer salutes and touches his hair with his hand. Surely he would have noticed his hair has become much longer, especially after he asked for a crew cut. (00:07:00)
Revealing mistake: In the drive room when Lister kicks the vending machine after he finds out he's going to die, you can see the shadow of the machine wobbling as if it were lightweight. (00:20:20)
Revealing mistake: When Rimmer runs to Lister after Lister unsuccessfully orders a black coffee, the camera tilts.
Revealing mistake: When the future echo Rimmer leaves the drive room and the actual Rimmer walks in, there is a visible line caused by the split screen effect, done to show two Rimmers onscreen at the same time.
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: 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: 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: 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)
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)
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)
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)
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)
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)
Confidence and Paranoia - S1-E5
Revealing mistake: When Confidence is trying to get Lister to take his helmet off, Lister climbs over a railing and jumps around him. It is clear that the footage of Lister jumping around was slowed down to make his movement appear as if there is a lack of gravity. (00:26:35)
Revealing mistake: When Lister goes into Rimmer's new room, there is a slight glitch with the split screen. When Lister is laughing at Rimmer's newspaper headlines on the door, the camera changes and the first frame only has Lister and Rimmer with the scutters. In the next frame the second Rimmer appears and the scutter on the right has moved. (00:06:10)
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