Revealing mistake: Holly's night cap is in front of the edges of his monitor. It should fade into the white border of his screen. (00:05:25)
Revealing mistake: During filming of the this episode, Craig Charles was called away for the birth of his son, Jack, so production manager Mike Agnew stepped in. Although Lister is wearing a space suit at the time you can still tell which part is Mike thanks to a continuity error - Mike forgot to put on Lister's plaster cast. (00:15:15)

Revealing mistake: When Lister uploads his memories, the wide shot shows various videos of Rimmer in the background, but in the close up it shows a plain white screen. (00:17:55)
Revealing mistake: The square bluescreen outline can be seen in the wide shot of the obervation dome scene. (00:24:50)
Revealing mistake: When Rimmer and Lister are in the observation dome, background stars can be seen on Lister's locks and on the top of his forehead. (00:25:40)
Revealing mistake: When Lister and the Cat drop the gravestone and break their feet, you can see that it does not land anywhere near Lister's left foot. (00:26:55)
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