When the female Rimmer is telling the male Lister that she hopes he gets pregnant, one of the headlines tacked up on Rimmer's bunk reads, "Arnold does it best" when it should read "Arlene does it best".
When the male Lister and Rimmer are in the sleeping quarters of the parallel universe Red Dwarf, a certificate for Arnold Rimmer is on the wall by the window, when it should read Arlene Rimmer.
The dream recorder wobbles.
How does Lister get pregnant? In the parallel universe when Lister was saying it was impossible to get pregnant, the female Rimmer says in her universe the physical law applies and Lister would therefore have the equipment to get pregnant. But at the end Lister goes back to their regular universe and therefore should no longer have the equipment to get pregnant; their universe's physical law would apply.