Out Of Time - S6-E6
Visible crew/equipment: When Kryten walks into the cockpit to offer Lister a beer, a boom mic can be seen at the top of the screen by Kryten's head. (00:10:05)
Visible crew/equipment: When Rimmer is speaking to Lister about how Ace Rimmer got all the breaks in his life, a boom mic can be seen in the top left of the screen. (00:24:15)
Emohawk - Polymorph II - S6-E4
Visible crew/equipment: After the crash when Lister comes in from outside, as he says 'it's waist high out there', a boom mic can be seen at the top of the screen. (00:09:30)
Out Of Time - S6-E6
Visible crew/equipment: At the start of the episode when Rimmer says 'any objections?' a boom mic can be seen moving across the top of the screen. (00:01:25)
Visible crew/equipment: When the Cat shoots the locker which the low Lister is hiding at The End of the episode, a boom mic can be seen above the Cat. (00:25:40)
Gunmen of the Apocalypse - S6-E3
Visible crew/equipment: Just after the simulant beams aboard and Lister and the Cat sit up from the table, a boom mic can be seen at the top of the screen. (00:07:40)
Visible crew/equipment: At The End of the episode, when Kryten explains why the Cat would kill himself, a boom mic shadow can be seen moving across his face,. (00:24:55)
Gunmen of the Apocalypse - S6-E3
Visible crew/equipment: After Rimmer is sick, as Lister comes to his rescue, a boom mic shadow can be seen on the wall behind them near the horns. (00:18:50)
Visible crew/equipment: When Holly says she is broadcasting on a higher frequency, when the crew are about to commit suicide, a boom mic can be seen in the top left of the screen. (00:23:30)
Gunmen of the Apocalypse - S6-E3
Visible crew/equipment: When sheriff Kryten asks for a drink, a boom mic can be seen at the top of the screen. (00:13:55)
Gunmen of the Apocalypse - S6-E3
Visible crew/equipment: At the start of the episode when Rimmer says they should surrender, a boom mic is visible at the top of the screen. (00:06:10)
Visible crew/equipment: Just after Lister says the despair squid will either kill them, eat them or hump them, a boom mic shadow can be seen moving on his face. (00:06:05)
Gunmen of the Apocalypse - S6-E3
Visible crew/equipment: When the crew's special skills get erased, a boom mic shadow can be seen on Lister. (00:24:20)
Visible crew/equipment: At The End of the episode when Lister makes his speech, the first shot shows a close-up of his face, then it shows a wide shot. A boom mic is pulled out of shot at the top of the screen. (00:25:50)
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).
Visible crew/equipment: When Lister puts his mutton vindaloo on the DNA modifier, the pole used to make the vindaloo float can be seen when the beam is cast on the dish. (00:22:55)
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