Other mistake: The Cat Priest has no fangs when every other Felis Sapiens has them.
Continuity mistake: When the book "Roots of Coincidence" is first seen it's a paperback but later in the episode it's suddenly a hardback.
Visible crew/equipment: When both Rimmer and his future echo walk through the door from the captain's office, the edge of the set can be seen. (00:15:09 - 00:16:04)
Visible crew/equipment: When Rimmer and Kryten invites Snacky to come with them to Red Dwarf, the reflection of a crewmember is visible in Snacky's head dome. (Note this is not Rimmer's reflection since there is no H).
Factual error: At the very end of the episode right before the Dwarfers sees Jesus and Judas, there is a plastic cup on their table. The episode is set in 23 AD.
Continuity mistake: When Rimmer shows Lister the luck virus and sexual magnetism virus, there is less than half of the liquid shown in the test tube for the sexual magnetism, but when Rimmer is about to go to Hollister's dinner later on, there is more liquid in the test tube. There was also only one test tube of sexual magnetism for Rimmer to use. (00:12:15 - 00:18:10)
Continuity mistake: When Lister tells Cassandra he is not going to kill her, he sticks his chewing gum on the wall and begins walking out. Seconds later, when the fan causes the gum to fly across the room, Lister is now standing inside the room. (00:27:15)
Continuity mistake: After the Cat has performed his dance routine, he begins to smoke a cigar and puts it down. The shot cuts to the ground controller, but when the shot cuts back to the Cat, the cigar is back in his mouth and has already been smoked. (00:10:30)
Continuity mistake: When Kill Crazy and Baxter are about to beat up Rimmer and Lister, as Baxter says "Say goodbye to your teeth", Kill Crazy has his hand pressed against Rimmer's jaw, advancing his face towards Rimmer's, but when Baxter goes to punch Lister, Kill Crazy's hand is by his side and is stood away from Rimmer. (00:19:50)
Continuity mistake: When Baxter tries to punch Lister the first time and fails, Kill Crazy is looking at Lister with his arms by his side, but in the next shot, Kill Crazy is now looking at Rimmer and doing motioning actions with his arms and face. (00:19:55)
Continuity mistake: When Thornton places the bracelet on Lister's arm, Lister's arm is at a straight angle, but in the next shot, his arm is at a rotated angle. (00:12:15)
Continuity mistake: The view of space outside the window in Rimmer and Lister's bunk shows a completely still background, but every other view of space through a window shows the ship moving. (00:14:10)
Revealing mistake: When the ship crashes into the snow planet, a wire can be seen attached to it as it crashes.
Factual error: When Kryten revives Lister with the defibrillator paddles, he places them on the clothing he is wearing. Defibrillator paddles need to be applied to bare skin for the electricity to get through. (00:27:35)
Revealing mistake: When Lee Harvey Oswald is preparing to shoot JFK on the fifth floor of the book depository, there is a shot of him from the side where he points his gun out of the window. To his left are a numerous boxes belonging to the book depository. Many of the boxes have "books" written on them, and some have hand written numbers on them. Later in the episode, the crew go back in time to send Oswald up to the sixth floor, but in the side view shot where he points his gun out of the window this time, the boxes are completely identical to the ones used on the fifth floor. The same set was likely reused.
Continuity mistake: When the crew are in the cockpit when Ace Rimmer is entering their dimension, Rimmer hopes that it is not him. When he says this, there is smoke around him, but this smoke was not there in the previous shot. (00:10:45)
Continuity mistake: At the start of the scene where the crew are giving Rimmer a funeral, Lister is shown clutching his "tie", and turns to Kryten, but in the next shot, he is suddenly looking up at the ceiling. (00:24:25)
Continuity mistake: When the crew meet the alternate dimension crew, both crews are standing side by side, but when the two Listers shake each other's hands, the Cat and Kryten from both crews have disappeared.
Continuity mistake: In the Aigburth Arms, when Frank hands the box with baby Lister to the barmaid, the barmaid takes the box placing her hand on top of the box, but in the next shot, her hand is suddenly on the side of the box.
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