Plot hole: If everything goes backwards, shouldn't Kryten and Rimmer have got fired, done their job and then got hired? Rimmer & Kryten discovered that everything was backwards in the cafe earlier in the episode. At the club they did their act and then got fired. Put this 'forwards' and it becomes wrong.
Revealing mistake: When Rimmer stands up from underneath the table, just before he says, "Thanks for your support" to Lister, listen very carefully and you can hear the stunt director saying "Action!" backwards. This shot was the splice point between the reversed and the regular footage. (I know this mistake is listed as being in the 'backwards version' on the DVD, but it is in the regular broadcast version too - you just have to listen harder for it.)
Other mistake: It is established in the backwards version of Earth, that all writing is reversed and is spelled backwards. However, there are some inconsistencies, as in some instances the writing has been mirrored to appear reversed, whilst in some instances the writing has just been spelled out backwards. For example, car number plates and signs on display in the background have been mirrored to appear reversed, but the newspaper that Kryten reads, the Nodnol sign and the Sensational Reverse Brothers poster appear normally typed out, and not as the mirror image that other aspects of this world have.
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