Revealing mistake: When the two dinosaurs are fighting besides the cliff, Malone and Jennifer Holmes hide behind a rock overlooking the cliff. One of the dinosaurs sweeps its tail, and knocks the two lovebirds over the cliff, but they fall onto a rock jutting out from the cliff. After we see them fall, there is a wide shot of the dinosaur swinging its tail. He swings it one way, then the other. After he swings it the first way, the tail freezes in mid-air for a split second, then swings the other way. It's obviously the same shot of the first tail swing, but played in reverse with a pause in between.
Revealing mistake: Whenever the dinosaurs are seen walking on dirt or sand, if you look closely, you can see individual grains of sand and small clumps of dirt. This would not be possible, unless the "dinosaur" was a foot and a half long at the most.
Revealing mistake: When the dinosaurs roll off of the cliff at the end of the fight scene, there's a closeup shot of Malone and Jennifer Holmes, and the dinosaur's tail as it falls. The problem is that it's obviously stock footage from the previous shot when the dinosaur's tail knocks Malone and Jennifer off of the cliff.
Character mistake: At the end, Professor Challenger misidentifies the dinosaur in the boiling water as a T-Rex. Its arms are too long, and it sports horns. It is much more likely a carnotaur. This area being a forte of his, I doubt Professer Challenger would make this mistake.
Revealing mistake: When the two dinosaurs are fighting, they don't seem to be of any weight. They're banging against the ground, and none of the trees shake, and almost no sand is disturbed. Also there is no loud gutteral banging from them hitting the ground. There is no indication that they are any heavier than five pounds on a cheap miniature set.
Revealing mistake: We all know that the dinosaurs FX in this movie are achieved by glueing horns, gills, and spikes to modern day lizards and reptiles, like baby alligators and the kinds you would buy at the pet store. But sometimes, it's too obvious. Especially in the fight between the two dinosaurs. The one sports a triceratops-like hood, which in real life was made out of a bony substance. But in the movie, it's wobbling like flimsy rubber.
Revealing mistake: During the fight between the two dinosaurs, one of the dinosaurs swings its tail, knocking Jennifer Holmes and Malone over the cliff, then there's a wide shot of its tail swinging. The trouble is that when the tail hits the two lovebirds, it changes color from the wide shot due to bad matting effects.
Visible crew/equipment: The string holding up the "Fire Monster" to make it appear like it's standing upright is seen in a couple of shots.
Revealing mistake: You don't even have to look closely to see that when Malone and Lord Roxton are fighting, none of their blows land home, but there is the (unrealistic) sound of punches, and the (overdone) reactions of the actors.
Plot hole: In the arial shot of the plateau, it's obvious its just a mesa. It's not at all large enough to support such big animals like those dinosaurs, who would have to eat great amounts of food. All small animals would be hunted into extinction, and all of the trees would have been cleared out long ago. Plus, there would be no place for the dinosaurs to hide, so the team should have seen the monsters right away.
Deliberate mistake: In real life, the Brazilian natives do not wear clothes at all except for speedo type shorts, even the women. This is a deliberate error to keep the film "clean."
Revealing mistake: When Frosty the poodle finds the first dinosaur, we can see that one of the horns he sports is crooked. Um, this didn't happen in real life.
Continuity mistake: When the dinosaurs are fighting near the cliff, their position relative to the cliff, and their position relative to each other, changes between shots.
Revealing mistake: The group closes a rock door to escape from the native Indians, but you don't even have to look closely to see the rock door is paper mache.
Factual error: The plateau blows up because of a volcanic explosion, but the explosion looks nothing like a real eruption. First of all, the plume would be a darker colour, mostly dark grey, and second, the explosion would fly out of the tube the group came out of, so they should still be running, not resting. Also, there would be danger from pyroclastic flow, and lava runoff.