Quarantine - S5-E4
Revealing mistake: As pointed out by Danny John Jules on the DVD Commentary, during the 'Feckles, Heckles, Hackles, Schmeckles' argument, Cat pushes his face right against Kryten's, leaving a brown makeup smear on Kryten's forehead. The mark is gone in the next shot. (00:19:45)
Quarantine - S5-E4
Visible crew/equipment: When the Cat tries to attack Rimmer in Quarantine, a boom mic can be seen at the top of the screen. (00:16:45)
Character mistake: After the crew realise that the matter paddle has made 2 copies of Red Dwarf, Kryten says something along the lines of, "When I threw the machine in reverse...", but it wasn't actually him who threw the machine in reverse, it was Lister. (00:06:55)
Continuity mistake: Just before Kryten chloroforms Lister, Lister's locks jump from behind his back to over his left shoulder between shots. (00:23:40)
Quarantine - S5-E4
Continuity mistake: After Lister gets the luck injection for the first time, he is holding his hat. In the next shot the hat is gone. (00:12:15)
Audio problem: When Kryten says 'Excuse me, could I just distract you for a brief second?' his lips are out of sync with the dialogue and continue to move after he has finished speaking. (00:23:35)
Continuity mistake: During the Higher Cat and Rimmer's dance when Kryten says 'and agony in torment' his arms are by his sides. In the next shot, his right arm is raised. (00:11:40)
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)
Terrorform - S5-E3
Continuity mistake: Just after one of the hooded legions reads from the scroll, he manages to turn around and face the opposite direction in one camera cut. (00:10:45)
Terrorform - S5-E3
Visible crew/equipment: A boom mic shadow can be seen along the bottom of the screen when the crew have to leave Starbug and walk. (00:11:25)
Terrorform - S5-E3
Visible crew/equipment: When Lister, Cat and Kryten leave the cockpit with the plan to make Rimmer feel good about himself to escape, a boom mic can be seen in the top left corner of the screen. (00:22:45)
Continuity mistake: Lister says his ID code is Lister-D-00169, yet in Furture Echoes he gave it as RD-52169. (00:14:55)
Visible crew/equipment: At the start of the episode, when Lister and Kryten talk about Lister's comic, a boom mic shadow can be seen on Kryten's head. (00:02:10)
Continuity mistake: After Rimmer and Nirvanah have sex, as they're getting out of bed, Nirvanah ties her robe twice. In the next shot as she's standing, the robe is open again. (00:14:00)
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