Emohawk - Polymorph II - S6-E4
Revealing mistake: When the crew get hit by the gas at The End of the episode, you can see that they are slightly moving. (00:27:30)
Rimmerworld - S6-E5
Revealing mistake: When the Cat says 'All in all a hundred per cent successful trip' you can see Lister mouth the line. (00:13:45)
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)
Rimmerworld - S6-E5
Revealing mistake: When Rimmer kisses the female Rimmer, the scar on his cheek disappears. (00:22:30)
Out Of Time - S6-E6
Revealing mistake: When the crew go through the unreality pocket that makes Starbug disappear around them, part of the Cat's face disappears into the background. (00:12:05)
Out Of Time - S6-E6
Revealing mistake: When the crew wake up from stasis, there is a model shot of Starbug flying towards the epicentre. You can see the legs of Starbug partially disappear as it gets closer to the ship. (00:12:45)
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