Revealing mistake: When Lister and Rimmer first meet their femaile opposites, Lister takes a step back and brushes against Rimmer's hologramatic shirt. (00:10:30)
Stasis Leak - S2-E4
Revealing mistake: When Rimmer hides under the table, he is not visible beneath the table when Lister and Rimmer walk in. (00:18:40)
Revealing mistake: After Starbug lands on the 'high' Red Dwarf, Rimmer is running out of power and starts to fade in and out. However, Cat, who must be standing too close to Rimmer also fades, the backdrop being partially visible through him. (00:08:05)
Revealing mistake: When Kochanski is on the phone falling through space, her lips do not match what she is saying. (00:25:55)
Emohawk - Polymorph II - S6-E4
Revealing mistake: The airlock opens before Ace activates the controls. (00:24:25)
Revealing mistake: In the observation dome, background stars are visible on Lister's hair and neck. (00:10:05)
Revealing mistake: When Lister kicks the simulant in the head, you can see he misses him by about a foot. (00:20:00)
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)
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)
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)
Revealing mistake: At the very start of the episode, when the SSS Esperanto is seen, the bubbles from the seabed are sinking rather than rising, revealing the piece of footage is being played backwards. (00:00:35)
Revealing mistake: When Lister and Rimmer are talking in the dome, the machine behind Rimmer keeps wobbling. (00:09:55)
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