Continuity mistake: When Lister takes the milk from his locker, keep an eye on his duvet. In the next shot when he pours the milk, it has moved and is hanging down from his bunk. The red mug on the table also moves from near the magazines to near the ashtray on the other side of the table. (00:14:35)
Other mistake: At the start of Rimmer's exam, the invigilator orders the students to "turn over their papers and start". Rimmer reads one side of the paper, then turns it over to read another question. The back of the paper should have been blank as otherwise there would be no point asking the students to keep the papers face down before reading the questions. (00:15:30)
Continuity mistake: Half way through the episode, the captain's office becomes covered by a wall. (00:24:55)
Audio problem: When Lister and Rimmer see Cat just after Holly explains how he evolved, Rimmer runs at Cat screaming before he passes through him and we hear a crash off screen to indicate Rimmer has just run in to something. He is a hologram and is not solid, so no sound should be heard. [The original version did not have this mistake. The sound effect was added for the now defunct Remastered series.] (00:27:10)
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