In the scene where Ed Malone and Jennifer Holmes meet up in the forest before the "Brontosaurus" chases them, Ed Malone says dialogue, but his mouth doesn't move. Also, Jennifer's mouth moves, but no dialogue comes out.
When the party of adventurers are in a boat approaching the dock where they will meet their helicopter pilot, Challenger is ostentatiously holding his large black umbrella up as a parasol against the tropical sun. However, the angle of the sun is such that no part of him is shaded by the umbrella.
In the exterior shots of the helicopter, Manuel Gomez can be seen in a tiny cockpit on top flying it (this also houses much of the rotor engine). When they approach the plateau, the interior cockpit expands to accommodate the Professor and the rest of the cast.
In the long shots of the helicopter, we can see that there are four side windows, two to a side. The forward windows are flanked by broad orange stripes which run to the edge of the rear windows. However, in the closeups of the characters peering out through three different windows, the orange stripes have vanished.
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.
When the camp is attacked, after the explorers have run away, Challenger identifies the creature as a Brontosaurus, yet the posture of the animal is all wrong and it has certain features from other herbivorous dinosaurs like Triceratops and Stergosaurus.
In the scene were the native girl waits for Jennifer to fall asleep, she tries to escape. In the scuffle to subdue her one of the large boulders is moved by an actor. Such a large rock, when bumped, could not have moved.