Continuity mistake: When the Cat is holding up the haddock, going through the people who have commited suicide, the shot changes and the haddock disappeares from his hand. Also after the shot changes, the Cat is then pointing at Kryten and smiling. (00:03:25)
Visible crew/equipment: Just before Red Dwarf explodes, there is a shot of Lister. In the background you can see a moving shadow of a crew member on the floor. (00:04:25)
Audio problem: When the high Kryten picks up the bomb, the high Cat can be seen to start speaking but no sound is heard. In the next shot, the high Cat is then smiling. (00:13:30)
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)
Terrorform - S5-E3
Continuity mistake: When Lister and the Cat type into the computer, the words are out of synch with the key tapping. (00:05:15)
Continuity mistake: When Lister says 'We don't exist here anymore,' the way Kryten is facing changes between cuts. (00:15:10)
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: When Lister slams his nose into the door of the cabinet, you can see it is nowhere near it. (00:17:50)
Continuity mistake: When the Cat says, "So what do we do with them?" Kryten disappears from Lister's left between camera shots. (00:17:30)
Continuity mistake: When the Cat is holding up the haddock, Kryten is stood in front of him holding out the scanner. In the next shot, Kryten has vanished, then reappears. (00:03:00)
Quarantine - S5-E4
Continuity mistake: When Rimmer is looking at the crew through the viewscreen in Quarantine, the reflection of the crew in the glass changes between shots. (00:21:20)
Continuity mistake: When Starbug jolts for the first time, Rimmer is looking to his right. The camera cuts and he is looking forwards. (00:03:30)
Plot hole: Allman does not remember his trial when The Inquisitor erases him, yet Lister and Kryten do remember their trial when they are sentenced. (00:01:20 - 00:13:30)
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)
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)
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