Revealing mistake: Just after the Trojans' conversation, there are two jolts. Lister jumps back too soon on the second jolt. (00:03:30)
Terrorform - S5-E3
Revealing mistake: At the start of the episode, Kryten's real wrist can be seen just before he removes his hand. (00:02:30)
Quarantine - S5-E4
Revealing mistake: When Lanstrom corners Lister, Kryten and the Cat, she is holding onto the fence. As she is a hologram, this would be impossible. (00:10:10)
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
Revealing mistake: When Lister uses the keypad to escape from Quarantine, more button lights turn off than he has actually pressed. (00:23:35)
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)
Revealing mistake: The pyrotechnic charges are visible under the 'High' Cat and Kryten's robes, before they are shot. There is also a wire trailing behind Kryten and running up his left leg, presumably a means of detonating the charges. (00:12:45)
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)
Revealing mistake: When Lister slams his nose into the door of the cabinet, you can see it is nowhere near it. (00:17:50)
Revealing mistake: At the very start of the episode, when the SSS Esperanto is seen, the bubbles from the seabed are sinking rather than rising, revealing the piece of footage is being played backwards. (00:00:35)
Revealing mistake: During the car chase, the box Rimmer is sitting on keeps moving. (00:21:25)
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