Continuity mistake: At the start of the episode when the Nazi commander is speaking to Ace, in all wide shots his left arm is wrapped around the crocodile, but in all close up shots his arm is by his side. (00:00:45)
Continuity mistake: When Rimmer finds the crew sneaking out to go fishing, he is stood facing away from them with his arms folded. In the next shot his arms are behind his back. (00:09:00)
Plot hole: The alligator that falls out of the plane with Ace hits the ground several minutes after Ace does. (00:04:25)
Emohawk - Polymorph II - S6-E4
Visible crew/equipment: When Rimmer picks up the gun just after the Cat turns into Duane Dibley, a boom mic arm shadow can be seen moving past the crates, just as Rimmer turns his back on the camera. (00:20:50)
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)
Continuity mistake: When Rimmer is talking about Lisa Yates to Lister, his tie is undone. When it cuts to the black box camera, his tie is correctly tied. (00:22:10)
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)
Continuity mistake: During Lister's exam, he takes the dough and puts it into the bowl while talking to Kochanski. When she turns into Rimmer, Lister is still rolling the dough and it is not in the bowl. (00:27:00 - 00:28:25)
Visible crew/equipment: When Lister is being taken over by The Inquisitor, a boom mic shadow can be seen moving on Kryten's chest. (00:04:15)
White Hole - S4-E4
Continuity mistake: Kryten takes the computer slug out of the machine with his left hand yet in the next shot it is in his right hand. (00:20:00)
Out Of Time - S6-E6
Visible crew/equipment: When Starbug hits the unreality bubble that makes the cat invisible, a boom mic can be seen just as the shot changes. (00:10:50)
Quarantine - S5-E4
Continuity mistake: When Lister gets the second luck virus injection, his locks move from in front of his shoulder to behind his back. (00:23:25)
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