Continuity mistake: When Rimmer shows Lister the luck virus and sexual magnetism virus, there is less than half of the liquid shown in the test tube for the sexual magnetism, but when Rimmer is about to go to Hollister's dinner later on, there is more liquid in the test tube. There was also only one test tube of sexual magnetism for Rimmer to use. (00:12:15 - 00:18:10)
Continuity mistake: After the Cat has performed his dance routine, he begins to smoke a cigar and puts it down. The shot cuts to the ground controller, but when the shot cuts back to the Cat, the cigar is back in his mouth and has already been smoked. (00:10:30)
Continuity mistake: When Thornton places the bracelet on Lister's arm, Lister's arm is at a straight angle, but in the next shot, his arm is at a rotated angle. (00:12:15)
Continuity mistake: At The End of "Nanarchy", Kochanski's hair is brown. However, throughout series 8, her hair is blonde. And with everything going on in "Nanarchy" there would have been nowhere near enough time for her to dye it.
Terrorform - S5-E3
Continuity mistake: When Kryten cuts off his hand, all of the fingers on the hand are straight and level with each other, but when he goes to attach his eye to one of the fingers, the middle finger is suddenly raised. (00:02:35)
Revealing mistake: When it is revealed that the DNA modifier has made Lister smaller, Rimmer, Cat and Kryten all have shadows cast on the floor, but Lister does not. (00:25:25)
Timeslides - S3-E5
Continuity mistake: When Kryten notices the photo of the bike in the countryside is moving, the peg attached to it is near the middle, along the top, but in the shot from behind, the peg is much closer to the right. The same goes for the photo of Kryten's birthday party. (00:03:30)
Continuity mistake: When Lister is telling Rimmer that he is going to die and that Rimmer doesn't need food, there is snow over his face, hair and coat collar, but a few seconds later, the snow has mostly disappeared. (00:09:35)
Continuity mistake: When Kryten is piloting Starbug and gently "eases forwards", there is a shot shown from behind Starbug where it is driving out of the cargo bay to the exit, and it is shown a couple feet off the ground. When Starbug exits Red Dwarf in the next shot, Starbug is suddenly much higher above the ground. (00:04:20)
Continuity mistake: Rimmer wears a hat with an antenna on the right hand side when on Starbug, and after arriving on Earth, but when he and Kryten are walking through the streets later on, the antenna is now on the left side of his hat, and is far more extended. (00:02:20 - 00:08:00)
Stasis Leak - S2-E4
Continuity mistake: In the hotel room, when future Lister is telling Lister he doesn't look pleased to see him, the two Listers are standing directly in front of each other, but from another angle, future Lister is standing around 90° rotated from before, and a few steps to the right of Lister. (00:25:35)
Continuity mistake: When Holly interrupts Rimmer learning Esperanto saying "The menu looks interesting", he appears on the TV screen, with his chin blocked by the edge of the frame. The shot then cuts to a view of the screen, and his entire face is suddenly fully in the frame. (00:05:00)
Deliberate mistake: When the computer has been launched, Lister walks into the drive room and notices the computer and stands behind Kryten and Rimmer. But when the camera is focused on the computer, there is no sign of Lister what so ever. (00:10:00)
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