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)
Continuity mistake: When Rimmer falls asleep whilst Lister is doing the jigsaw, there is a whisky bottle on the table. Later in the episode, when being viewed on the black box, the whiskey bottle is no longer on the table. Lister could not have thrown it away since he left the table to the Hologram Simulation Suite at The End of the scene. (00:10:20 - 00:19:35)
Continuity mistake: When Lister is doing the jigsaw and Rimmer starts singing in bed, the pint mug on the table has nothing in it. Later on, when Lister returns after giving Rimmer his memory, an inch of whisky has suddenly appeared in the bottom of the mug. (00:10:20 - 00:19:35)
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)
Stasis Leak - S2-E4
Continuity mistake: When Petersen walks away from Lister with the cases, the two women follow him. In the next shot the women have swapped places. (00:17:45)
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)
Continuity mistake: When Rimmer is talking about Yvonne McGruder, Lister is not wearing a hat. When we see the black box recording of the conversation, Lister is now wearing one. (00:08:25 - 00:16:45)
Revealing mistake: When Lister and the Cat drop the gravestone and break their feet, you can see that it does not land anywhere near Lister's left foot. (00:26:55)
Revealing mistake: Holly's night cap is in front of the edges of his monitor. It should fade into the white border of his screen. (00:05:25)
Continuity mistake: When Holly interrupts Rimmer learning Esperanto saying "The menu looks interesting", he appears on the TV screen, with his chin blocked by the edge of the frame. The shot then cuts to a view of the screen, and his entire face is suddenly fully in the frame. (00:05:00)
Stasis Leak - S2-E4
Continuity mistake: In the lift, when the woman in the video is explaining how the lift works, she mentions "a bag will drop containing sedatives and cyanide capsules", pointing upwards. The shot cuts to Lister, and the woman in the video can be seen to the left of shot, with her arms at her side. (00:08:35)
Stasis Leak - S2-E4
Continuity mistake: In the hotel room, when future Lister is telling Lister he doesn't look pleased to see him, the two Listers are standing directly in front of each other, but from another angle, future Lister is standing around 90° rotated from before, and a few steps to the right of Lister. (00:25:35)
Stasis Leak - S2-E4
Continuity mistake: When Lister is in the hotel room with future Lister and Kochanski, Lister says he came because he thought he was going to get married whilst talking to future Lister, but in the next shot, Lister's facial expression has suddenly changed and is now looking more confused. (00:25:35)
Stasis Leak - S2-E4
Continuity mistake: When future Lister is making Lister and Cat leave the hotel room, Lister begins talking to future Lister about living his life. Future Lister is standing beside the door in one shot, but has his head suddenly resting on the door in the next shot. (00:26:25)
Other mistake: The shots of Blue Midget driving back to Red Dwarf that are shown from behind are visibly the exact same shots used each time. This is evident as the driving movements of Blue Midget, its position in the frame, and Red Dwarf's position in the background are all identical each time.
Continuity mistake: When Rimmer is about to tell Lister how many times he made love, Lister tells him that he'll "wake up in the morning with a hangover." But Rimmer and Lister already have fallen asleep and woken up, and they have the hangovers right now.
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