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)
Continuity mistake: When Bird-man loses his glasses, he is feeling his face. In the next shot both of his hands are on the floor. (00:25:40)
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)
Continuity mistake: After Lister and Ace do the 'awooga' dance, Lister locks change from being clumped together to being mostly behind his back. (00:21:40)
Revealing mistake: When the Cat is pushing the trolly of cigarettes, Rimmer walks past him and bangs his arm against it. (00:09:30)
The Last Day - S3-E6
Continuity mistake: When Kryten packs his spare heads away, he places the first one in the case with his left hand. In the next shot it has moved to his right hand. (00:03:50)
Visible crew/equipment: At the start of the episode when the Nazi attacks Ace in the plane, a boom mic can be seen at the top of the screen. (00:00:55)
Out Of Time - S6-E6
Visible crew/equipment: In the medi bay when Kryten says, 'I'll have a talk with him droid to droid,' a boom mic is visible at the top of the screen. (00:05:55)
Revealing mistake: Just before the Cat starts his dance, Kochanski is at the back of the cockpit of Blue Midget and is bouncing up and down, yet the ship is not moving. When it starts moving, Kochanski is sitting still. (00:08:05)
Emohawk - Polymorph II - S6-E4
Visible crew/equipment: Just after the grenade explodes, a boom mic shadow can be seen on Kryten's head. (00:27:00)
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)
The Last Day - S3-E6
Continuity mistake: Lister's hands change position between cuts when Kryten walks out in the opening scene. (00:02:05)
Revealing mistake: When Holly gives Rimmer a new uniform, during the switch, Holly can be seen to jolt on his screen in the background. (00:03:20)
Stasis Leak - S2-E4
Continuity mistake: The second time we see the scene with Rimmer's head popping through the table, the line 'There, that wasn't too bad, was it?' is missed out. (00:04:40 - 00:19:20)
Stasis Leak - S2-E4
Continuity mistake: The second time we see the hologramatic Rimmer pass Rimmer and Lister in the corridor, two other people have appeared walking past. (00:04:00 - 00:18:25)
Stasis Leak - S2-E4
Continuity mistake: When Rimmer puts his head through the table, the magazines change position next to his head. (00:18:55)
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