Revealing mistake: As Lister tries to sharpen/shorten his nails after getting out of deep sleep, you will see just before he stops that one nail snaps at the finger. He continues as if nothing happens, holding his hand still, as if it was still in the sharpener. (00:02:00)
Other mistake: Kryten places Rimmer's light bee on the floor, then begins the process of reinitialising him. As the process finishes and Rimmer looks at Lister (who looks away), the light bee, which should be hovering within his torso projecting his image, is still visible on the floor between his feet. (00:06:00)
Continuity mistake: When Lister is watching the video message of a psiren, posing as Kochanski, Lister asks "How? I don't understand" and Kochanski, on the video appears to move her face beginning to reply, but when the shot cuts back to the video, Kochanski's face is suddenly still. (00:16:20)
Revealing mistake: Just after we see the first meteorite, there is a brief moment where the model shot and the crew walking into the cockpit overlap causing both images to be seen together. (00:16:55)
Revealing mistake: When Starbug gets hit by the meteorite, the word 'Starbug' on the ship is backwards revealing the film has been mirrored. (00:18:25)
Revealing mistake: The spike that kills Pete Tranter's sister is visible before she reacts to it, just below her elbow. (00:21:20)
Revealing mistake: During the 'Two Listers' scene, specifically the cut when Kryten tosses the apples to the Listers, the split screen is quite visibly moving in between the chairs. (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