Emohawk - Polymorph II - S6-E4
Revealing mistake: When the grenade explodes, the explosion is to the left of where the grenade landed. (00:26:50)
Out Of Time - S6-E6
Continuity mistake: When Rimmer picks up the bazookoid at The End of the episode and runs into the engine room, the gun is pointing up. In the next shot from inside the engine room, it is pointing down. (00:27:10)
Out Of Time - S6-E6
Continuity mistake: When the crew fire at their future selves, Lister says 'locking on, firing'. His locks are behind his back but in the next shot they are in front of his left shoulder. (00:26:10)
Revealing mistake: In several episodes, but mostly in series 6, we see lots of profile camera shots of just Lister and The Cat at the front of the cockpit in Starbug. Quite a few times you will see Lister mouth The Cat's lines so he knows when to start speaking and what to say.
Revealing mistake: Just after we see the first meteorite, there is a brief moment where the model shot and the crew walking into the cockpit overlap causing both images to be seen together. (00:16:55)
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)
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)
Gunmen of the Apocalypse - S6-E3
Visible crew/equipment: When the simulant has beamed on board Starbug, Lister and The Cat get up slowly from the table. Look closely, and you can see the shadow of a boom mic cast on Lister and The Cat. (00:07:50)
Rimmerworld - S6-E5
Revealing mistake: When the Cat says 'Form an orderly queue behind the gunsight' you can see Lister mouth the line. (00:15:40)
Other mistake: When Legion stabs his left hand, everyone grabs and shakes their left hands - except Lister, who grabs and shakes his RIGHT hand. (00:22:40)
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)
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)
Emohawk - Polymorph II - S6-E4
Continuity mistake: When Lister gives the Gelf the cigar, he looks over his shoulder at Kryten. In the next shot, he looks over his shoulder again. (00:11:50)
Rimmerworld - S6-E5
Visible crew/equipment: When Lister, the Cat and Kryten arrive on Rimmerworld, as the Rimmer guard steps out and says 'silence mutant' a boom mic is visible at the top of the screen. (00:21:05)
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