Author Existence Failure: The last novel, Skeleton Men of Jupiter, was never completed due to Burroughs' death in 1950. The existing sections of it were, however, published as a shorter story in the collection John Carter of Mars 14 years later.
Development Hell: The film of A Princess of Mars has been in on-and-off development since 1931. It eventually reached the big screen as John Carter in 2012, just in time for the 100th anniversary of the first book's publication.
There was a Direct-to-DVD movie by The Asylum that came out in 2009 and among other things made Barsoom into a planet orbiting Alpha Centauri (referred to as Mars early on), and John was a modern soldier who was involved in a teleportation experiment and was followed by an Afghan opium dealer who wanted the princess (played in this version by Traci Lords).
Science Marches On: Burroughs was just one of many sci-fi writers of his era that depicted Mars as a habitable planet filled with various exotic fauna as well as intelligent life. All those dreams were ruined by the Mariner 4 misson to Mars in 1964.
Also, in A Princess Of Mars has the guns using explosive ammonote The bullet breaks open on impact, and the contents explode when sunlight hits it. There's a 'non-explosive' version, for night raids where the ammo is expected to be left behind. powered by Radium, of all things. This gets Hand Waved in a footnote, however: The original notes use the 'Martian' name for the powder placed in the glass bullets and "Radium" is used as a place-holder, since he expects it to behave that way anyway.