Visible crew/equipment: When Lister downs his pint of wine, the shadow of a boom mic can be seen on the top left of the screen. (00:20:00 - 00:21:55)
Continuity mistake: When Rimmer is standing on the beach and conjures up the green car you can see the wheels are partly in the sand. When he then gets into the car, tyre tracks are visible behind it. When he starts driving off, the tyre marks are now gone. (00:18:25)
Visible crew/equipment: When Rimmer and his seven kids get out of the car, three crew members and a white production truck are visible in the reflection. (00:25:15)
Continuity mistake: When the Cat and Lister are playing golf, they stop next to a bed with Marilyn Monroe sitting on it. When the camera angle changes, they begin walking from the other side of the bed. Also Marilyn Monroe is now kneeling. (00:24:20)
Continuity mistake: As the crew enter the game, Lister pushes the door to the beach open. However in the next shot, the door is opening from the other side, i.e. Lister is pulling the door. (00:16:10)
Continuity mistake: When Lister puts his Better Than Life helmet on, the cable is hanging over his right shoulder. In the next shot it is hanging down over his left shoulder. (00:15:45)
Continuity mistake: When the bike appears on the beach, the front tyre is above the level of sand. In the next shot it has sunk by about an inch. (00:17:50)
Continuity mistake: The sand that lands on Rimmer's shoulder when Lister and the Cat drive away on the bike disappears in the next shot. (00:18:10)
Continuity mistake: When the Cat is kissing the photo of the mermaid, in the next shot the photo has disappeared. (00:24:50)
Revealing mistake: In the observation dome, the borders of the blue screen used to project the image of space behind Lister and Rimmer can be seen around the dome. (00:09:55)
Visible crew/equipment: When Rimmer is getting in the car on the beach, you can see the cameraman's reflection on the side of it. (00:18:25)
Visible crew/equipment: When the Cat licks Better Than Life's host, there is a shadow of a boom mic at the top of the screen on the wall. (00:19:35)
Other mistake: The tax letter and Rimmer's mother's letter are both already opened before Lister takes the letters out. As there is no one else aboard Red Dwarf, the letters would not have been checked by anyone so they should still be sealed. Also, not all of the letters are already open, just the two that Rimmer receives.
Revealing mistake: In the observation dome, background stars are visible on Lister's hair and neck. (00:10:05)
Continuity mistake: When Lister and the Cat put their sunglasses on, Rimmer's tie changes between camera cuts from over his shoulder to neat and straight. (00:18:00)
Revealing mistake: When Lister and Rimmer are talking in the dome, the machine behind Rimmer keeps wobbling. (00:09:55)
Continuity mistake: When Marilyn Monroe waves to the Cat on the beach, in the next shot her arms are by her side. (00:16:50)
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)
Continuity mistake: When Lister is questioning how they can remake Casablanca, he is holding the video tape, but in the next shot, the tape has disappeared from his hand and is touching the control panel. (00:04:40)
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)
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