Revealing mistake: When Kochanski is on the phone falling through space, her lips do not match what she is saying. (00:25:55)
Revealing mistake: When Kochanski is shot with the spear, the padding that gives the impression the spear is through her leg is visibly bulging in her trousers as ahe pulls on the rope. (00:26:50)
Revealing mistake: When the ship crashes into the snow planet, a wire can be seen attached to it as it crashes.
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