Terrorform - S5-E3
Continuity mistake: The cutting torch Kryten uses to cut his hand off appears from nowhere. (00:02:20)
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 Lister wipes down the biohazard cabinet, a boom mic can be seen in the top right of the screen. Also, just before the camera moves past the pillar, the entire microphone and arm can be glimpsed. (00:05:00)
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)
Terrorform - S5-E3
Continuity mistake: When we first see Kryten, there is no long grass anywhere near him. When Lister rescues him, it has appeared from nowhere. (00:01:35 - 00:06:10)
Continuity mistake: When the crew almost kill themselves, Kryten is holding the harpoon gun to his head. The point of the spear is missing but in the next shot it has reappeared. (00:23:30)
Continuity mistake: The bottle of chloroform Kryten uses to take Lister by surprise appears from nowhere. (00:23:30)
Revealing mistake: After Starbug lands on the 'high' Red Dwarf, Rimmer is running out of power and starts to fade in and out. However, Cat, who must be standing too close to Rimmer also fades, the backdrop being partially visible through him. (00:08:05)
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)
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