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)
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)
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)
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)
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)
Visible crew/equipment: When the alligator lands on the German guards, a fluffy grey boom mic can be seen at the top of the screen. (00:04:25)
Visible crew/equipment: If you look closely when the camera follows Lister (in the scene just before the meteor crashes onto Starbug) you can actually see the edge of the set and some studio lights. (00:04:10)
Rimmerworld - S6-E5
Visible crew/equipment: When Rimmer argues that they should not board the simulant ship, the shot of Lister in the foreground and Rimmer in the background shows a boom mic just above Rimmer's head. (00:04:40)
Continuity mistake: Kryten removes the polymorph underpants and throws them under the covers of Lister's bed. When he checks the bed and pulls out a snake the pants are now on top of the covers (even though he says they're gone) and the covers themselves are now arranged differently. (00:10:00)
Continuity mistake: At the disco, when the Rimmers are seated at their table, the plant moves from in front of the lamp to behind it. (00:15:40)
Confidence and Paranoia - S1-E5
Continuity mistake: When Lister is in the Medi-bay, he has his duvet on his lap. The angle changes and he has the duvet over his shoulders. (00:12:30)
Stasis Leak - S2-E4
Continuity mistake: At The End of the episode when Lister is talking to Holly on his watch, Lister's red sleeve is in line with the watch, but as the camera cuts to show Holly, the sleeve is gone. (00:24:10)
Revealing mistake: When Starbug gets hit by the meteorite, the word 'Starbug' on the ship is backwards revealing the film has been mirrored. (00:18:25)
Visible crew/equipment: When Rimmer is getting in the car on the beach, you can see the cameraman's reflection on the side of it. (00:18:25)
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