When they go down to the cargo hold and Kryten says "It's here" and "Somewhere", Robert Llewellyn speaks with his own English accent rather than Kryten's accent (it was one of the first scenes he filmed and he hadn't decided on how Kryten would speak)
When Lister opens the microwave to remove his kebab you can see the plate move, as the stage hand was still placing it into the empty space.
When the polymorph has turned into Mrs. Rimmer and is trying to make Rimmer angry, as she stands up from the table, you can see her bump into Rimmer. This should not be possible - Rimmer is a hologram, and therefore composed entirely of light; the polymorph, of course, is physical.
Rimmer is established to be a hologram who cannot physically interact with his surroundings. So how can the polymorph attach itself to Rimmer's forehead and suck out his anger?
At the start when Kryten is cleaning Lister's quarters, Lister seems surprised that Kryten has a groin attachment but the crawl at the start of Backwards states that Lister rebuilt Kryten and so he would know that this was there.
After Kryten takes off Lister's tiny boxer shorts and puts them in his bunk, he claims that they are not there as the polymorph has changed into a snake. However, the boxer shorts are clearly visible on the bunk as he says this.