Stasis Leak - S2-E4
Revealing mistake: In the opening, Holly gives an additional message saying "On our journey, we have encountered many strange and bizarre things. Only last month, we can across a moon which was shaped exactly like Felicity Kendal's bottom. We flew around that one a couple of times." Before Holly says "We flew around that one a couple of times", he can be seen about to say the line, but visibly hesitates, presumably waiting for his cue. (00:01:35)
Stasis Leak - S2-E4
Revealing mistake: When Lister, the Cat and Rimmer are stood in the shower and the Cat keeps saying 'What is it?' he turns to Rimmer and brushes against his tie throughout the scene. (00:10:05)
Stasis Leak - S2-E4
Revealing mistake: Rimmer, Lister and the Cat are not visible in the mirror when the crew member finds them in the shower. (00:10:25)
Stasis Leak - S2-E4
Revealing mistake: When the Cat gets jumped by the people in fancy dress, the red wall behind them wobbles. (00:17:50)
Stasis Leak - S2-E4
Revealing mistake: When Rimmer hides under the table, he is not visible beneath the table when Lister and Rimmer walk in. (00:18:40)
Stasis Leak - S2-E4
Revealing mistake: At The End of the episode when the future Lister closes the door on the Cat and Lister, you can see the partial editing out of Lister and the Cat as the door closes. As this happens, Lister and the Cat disappear before the door has passed them. (00:26:50)
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