Revealing mistake: In the scene where Kochanski, the Cat, and Kryten first discover the time wand, there are a group of Canaries who are frozen in time from its effects. In the first shot of these people, the woman on the far right of the screen blinks her eyes. (Confirmed on DVD commentary). (00:15:50)
Visible crew/equipment: When the crocodile is thrown at Ace, the crash mat is visible as he lands. The mat disappears in later shots. (00:01:00)
Timeslides - S3-E5
Continuity mistake: When we see the photo of the skiers, there are no boxes in front of the screen. When the shot changes, two boxes have appeared which Rimmer and Lister use as steps. (00:07:20)
Stasis Leak - S2-E4
Continuity mistake: When Lister puts his hand under his arm pit, his locks move from behind his back to over his right shoulder in one cut, and then back again. (00:18:00)
Quarantine - S5-E4
Revealing mistake: When Lister uses the keypad to escape from Quarantine, more button lights turn off than he has actually pressed. (00:23:35)
Other mistake: Kryten sees Ace's ship before it materialises. (00:12:55)
Stasis Leak - S2-E4
Continuity mistake: When the Cat attacks the 'dog' in the hotel, between shots it changes position as he jumps on it. (00:22:20)
Rimmerworld - S6-E5
Revealing mistake: When Rimmer kisses the female Rimmer, the scar on his cheek disappears. (00:22:30)
Other mistake: The milkshake and toffee crisp come out of a different vending machine to where they were ordered. (00:01:50 - 00:06:55)
Out Of Time - S6-E6
Visible crew/equipment: When the Cat becomes visible again after going through an unreality pocket, the shot changes to a wide shot of the crew. A boom mic can be seen to be pulled out of shot at the top of the screen by Kryten's head. (00:11:40)
Revealing mistake: When Legion gets hit, you can see his real face for a second when he adjusts his mask. (00:21:30)
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