Revealing mistake: When Rimmer and Lister are being forced out Captain Hollister's office due to the effects of the fight catching up with them, a rattling sound can be heard off-screen, indicating the use of a hidden trolley-like device pushing them into the wall. (00:21:55)
Continuity mistake: When Hollister picks up the lemon drink, his left hand is by his side. In the next shot it is on his chest. (00:01:50)
Continuity mistake: When Rimmer gives Hollister the directives to sign, he holds the clipboard in both hands. In the next shot his left hand is under the first page. (00:02:15)
Other mistake: When Rimmer tries to silence the vending machine, he covers up the input microphone with stickers instead of covering the speaker just above it. (00:05:05)
Visible crew/equipment: When the vending machine says it will destroy Rimmer, a boom mic can be seen at the top of the screen. (00:06:05)
Revealing mistake: When the Cat, Lister and Kryten are walking and the Cat is complaining about rock, keep an eye on the background. They start walking just before a set of steps, then do a complete loop back to the steps for no reason. (00:06:15)
Continuity mistake: Just before Rimmer and Lister drink the hooch, Lister's locks are behind his back, then one of the locks moves to in front of his left shoulder during a brief shot of Rimmer. (00:13:30)
Visible crew/equipment: After Rimmer faints after getting drunk, a boom mic shadow can be seen on the bedsheets. (00:15:45)
Visible crew/equipment: When we first see Rimmer and Lister in the medi-bay, the camera crew can be seen in the reflection of the tv. A boom mic arm can also be seen moving. (00:16:55)
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