Timeslides - S3-E5
Deliberate mistake: The Eagle on Hitler's briefcase is that of the (post war) Federal Republic of (West) Germany - not that of the Nazi German Third Reich. This was deliberate - showing the Third Reich Nazi emblem of an eagle holding a swastika would have had the episode banned in Germany, Austria, Poland and any number of other European countries. (00:08:25)
Other mistake: When Cat picks up the can of Leopard lager which is invisible to Lister, he shakes it, pops the cap and sprays Lister with foam. The foam is visible but the can remains invisible, and that makes no sense. If we are seeing the scene from Cat's point of view the can and foam should both be visible, and if it is from Lister's then neither should be.
Trivia: "Meltdown" was originally planned to be the first episode of Series IV of Red Dwarf. However, the militaristic tone of this episode - and in particular Dave Lister's strident anti-war speech near The End of the episode - meant it fell foul of the BBC censors. The original planned transmission date (Feb 14 1991) coincided with the outbreak of "Operation Desert Storm" - the Gulf War...and the BBC felt that an "anti-war" episode of Red Dwarf would be inappropriate for a country at war with Iraq.
Chosen answer: In-universe, after leaving Red Dwarf after his first appearance, Kryten managed to crash Lister's space-bike into an asteroid. Lister found his remains and decided to rebuild him, however, due to the high level of damage and Lister's questionable repair skills, he was unable to recreate Kryten exactly, leading to changes in appearance, personality and accent. In reality, the original actor wasn't available, coupled with presumably practical concerns about one-off makeup as opposed to makeup for a regular cast member, so some visual changes were made.
Tailkinker ★