Continuity mistake: At the start of the episode, it is clear that the capsule fully rotates and the "contents: 2" sign is not on the capsule. (00:01:05)
Continuity mistake: Kryten removes the polymorph underpants and throws them under the covers of Lister's bed. When he checks the bed and pulls out a snake the pants are now on top of the covers (even though he says they're gone) and the covers themselves are now arranged differently. (00:10:00)
Visible crew/equipment: When the bazookoid blasts hit the Polymorph near The End, you can see the stand that's holding the Polymorph at the bottom of the screen. (00:24:45)
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