In the long shots, the single tree on the small plateau is much taller than the gap between the two plateaus, but in the medium shot when they drop it across, it barely reaches the large plateau.
The Plateau in the film has been described as a world that is "cut off from evolutionary development." If that were true then dinosaurs from different eras would not be in the same place, nor would there be any ape-men or humanoids.
The newspaper headline announcing Malone's departure is dated in January. Right after this is shown, Malone types that he has been gone for less than three months and it is now December 12th.
A Chimpanzee is native to Africa, not in the Brazilian jungle.
Near the beginning, Malone says that dinosaurs have been extinct for 10 million years. Actually, the correct number is 65 million years.
In the first newspaper article, the word "accepted" is misspelled as "accpted."
At the beginning of their boat journey in the Amazon forest, a bird appears in the bushes at the right side of the screen. The bird is obviously thrown into the shot by someone.
During the Brontosaurus' rampage in London, as it approaches the statue, a large metal support used to keep the dinosaur model steady very visibly pops in and out of view for a couple frames.
Professor Challenger travels to the Lost World to prove his claims that dinosaurs still live, yet no one on the expedition seems to have brought a camera.