Revealing mistake: When Able screws on Kryten's head he only turns it once, yet in Tikka To Ride, Lister screws his head on at least twice around. (00:15:30)
Timeslides - S3-E5
Revealing mistake: When Rimmer dies at The End of the episode, a dummy is visible just before the explosion. (00:26:40)
Revealing mistake: When Holly is talking about his chess game, you can see his black clothing to the left of his chest, revealing he's hiding a body, and isn't just a computer generated floating head. (00:06:30)
Revealing mistake: When Holly gives Rimmer a new haircut, Rimmer salutes and touches his hair with his hand. Surely he would have noticed his hair has become much longer, especially after he asked for a crew cut. (00:07:00)
Revealing mistake: During filming of the this episode, Craig Charles was called away for the birth of his son, Jack, so production manager Mike Agnew stepped in. Although Lister is wearing a space suit at the time you can still tell which part is Mike thanks to a continuity error - Mike forgot to put on Lister's plaster cast. (00:15:15)
Revealing mistake: After the T-Rex eats the curry, it starts stamping about the cargo bay, yet there are no ripples in the very watery curry. (00:08:30)
Revealing mistake: After the crew transport themselves out of the book depository to escape the FBI, the astronaut helmet is transported afterwards and the two FBI agents start shooting at the box the helmet was placed on. When the box is shot, there are no bullet holes left on it.
Revealing mistake: When the Polymorph, posing at the snake, jumps on Lister, it is apparent that the snake on Lister has been replaced by a plastic inflatable replica that has no features and does not move. (00:10:05)
Revealing mistake: When the Outland Revenue guy breaks Rimmer's thumb with the mallet, the mallet does not touch his thumb, but instead his fingers. (00:28:35)
Stasis Leak - S2-E4
Revealing mistake: In the opening, Holly gives an additional message saying "On our journey, we have encountered many strange and bizarre things. Only last month, we can across a moon which was shaped exactly like Felicity Kendal's bottom. We flew around that one a couple of times." Before Holly says "We flew around that one a couple of times", he can be seen about to say the line, but visibly hesitates, presumably waiting for his cue. (00:01:35)
Revealing mistake: When Pree shows Lister his Medi-Bay file, it is a flipped shot. Note the danger sign next to the computer. (00:27:40)
Revealing mistake: The potatoes used in spud duty are obviously just cardboard cut outs used to give the impression that there are lots of potatoes. (00:19:25)
Revealing mistake: When Rimmer runs to Lister after Lister unsuccessfully orders a black coffee, the camera tilts.
Revealing mistake: When Lister goes to Rimmer's room the second time, the door opens before Lister hits the switch.
Revealing mistake: During the last scene of the episode, the Cat keeps brushing against Rimmer's hologramatic shirt. (00:27:10)
Revealing mistake: When Lister uploads his memories, the wide shot shows various videos of Rimmer in the background, but in the close up it shows a plain white screen. (00:17:55)
Revealing mistake: When the simulant hits Able over the head with the steel bar, you can clearly see it bend. He hits him a second time a few moments later and it is straight again. (00:14:45)
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