Revealing mistake: When Pree shows Lister his Medi-Bay file, it is a flipped shot. Note the danger sign next to the computer. (00:27:40)
Deliberate mistake: When the computer has been launched, Lister walks into the drive room and notices the computer and stands behind Kryten and Rimmer. But when the camera is focused on the computer, there is no sign of Lister what so ever. (00:10:00)
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