In the museum when Challenger is giving his lecture, the camera zooms out of a carnivorous dinosaur's mouth. The dinosaur is mounted in a horizontal, not vertical standing, which would have been correct for the period. Also, Challenger and Summerlee comment on the live dinosaurs on the Plateau as standing in a different way than originally conceived. This makes no sense if they have a skeleton set up that way in their museum.
The dinosaurs are severely inaccurate. The T. Rex's arms are too big and have three fingers and Summerlee identifies a certain dinosaur as a Brontosaurus early on in the movie, but no such creature existed, also, if it did, it would not have spikes on it's tail like a Stegosaurus.
When the explorers enter The Lost World out of the Mongolian winter snow, their breath is still visible in many shots even though it is supposed to be tropical.
The film takes place in 1938, but the characters are able to identify Quetzalcoatlus and Eoraptor. These didn't get their names until 1975 and 1993, respectively.
Amanda White refers to "the endangered species list". The film takes place in 1938; the first endangered species list was created in 1966.