Other mistake: Cassandra says that all the canaries will be dead in 1 hour, with the exception of Rimmer, Kryten, Lister, Cat and Kochanski. However, Kill Crazy was still knocked out in the transport vehicle and presumably would have survived too.
Factual error: While it is true that lithium carbonate can be used to treat depression and other psychiatric disorders, it cannot be administered in gaseous form. It is a very stable salt and only starts to form vapour at 1,310 degress Celcius. Spraying it about as a 'mood stabliser' at that temperature, as Kryten does, is going to kill everyone on the spot.
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.