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: At the start of the episode when we first see the polymorph, it looks into the mirror, then covers its eyes. Yet when we see the reflection, it already has its hands over its eyes before it gets to the mirror. (00:01:30)
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)
Factual error: The polymorph goes through a metal pipe and a tap which bulges and becomes mis-shapen. The tap then changes back to its original shape, which is impossible. (00:18:00)
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)
Revealing mistake: When the Polymorph, posing at the snake, jumps on Lister, it is apparent that the snake on Lister has been replaced by a plastic inflatable replica that has no features and does not move. (00:10:05)
Continuity mistake: When the group are about to open the door with the heat seeking missiles inside and the Polymorph sneaks up behind them, Lister is standing at the front of the group, Cat and Kryten are behind Lister and Rimmer is standing at the back, behind all three. In the next shot, as the missiles exit the door, Rimmer is now suddenly standing front of Cat. (00:24:40)
Continuity mistake: When the Polymorph is switching between various appearances at the start, it stops at a rabbit. When the Polymorph stops at the rabbit, there is a damp stain beside the two yellow lines on the floor. This damp stain was not there when the Polymorph was switching between other forms. The stain just suddenly appears out of nowhere. (00:01:45)
Audio problem: When Kryten starts looking over Lister's bunk trying to find the Polymorph, posing as the boxers, the boxers, and therefore the Polymorph, are visible on the bunk. The sound of the Polymorph transforming is heard, indicating the Polymorph has changed form, but the boxers remain there, without any transformation. It is not until a few shots after that the Polymorph has appeared to have changed form. (00:10:00)
Character mistake: When Rimmer, Cat and Kryten go down to the cargo hold to search for the Polymorph, Kryten says "It's here" and "somewhere", but he delivers the line in an English accent and not in his robot voice. (00:13:15)
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