Revealing mistake: A boulder tumbles towards James Mason and his party though a cave. With all the smashing against the walls and floor of the cave, the boulder stays entirely intact. However, just before the boulder hits a wall with two tunnels while flying through the air, the boulder now shows two majors fractures that were unseen earlier. The fractures were added to the boulder to give it a more dramatic impact by it breaking up with one piece nearly striking James Mason. (00:55:50)
Continuity mistake: After the villain (Thayer David) kills a duck, a boulder begins to move and brushes the side of his right leg as seen in a close-up and both legs can squarely be seen as he is standing beside the rock. The scene changes to a stunt-man who has his left-side towards the camera and this time his right leg is closer to the boulder which pushes him over a ledge. (01:50:35)
Continuity mistake: James Mason meets the villain (Thayer David) who holds a gun on Mason and the rest. Mason outwits him by throwing salt into his face to blind him. When the salt hits his face, he closes his eyes (too early) before impact. The salt impact hits the right side of his face. An angle change occurs and we see a close-up of him (or a stunt double - his hair is different) and the salt hits the actor squarely in his face with his eyes open to the last microsecond. The scene changes back to show Mason and the villain with the salt on the right-side of the villain's face again with him in agony. (01:24:20)
Revealing mistake: When the first dimitridon (the sail-finned dinosaurs) shows up, the dimitridon's face changes color from the wide shot and the closeup of it and the Professor and the Count because of bad matting effects.
Revealing mistake: The actors get somewhat ragged as they spend months traveling towards the center of the Earth. But all of them are quite well shaven and their hair seems to never grow. Arlene Dahl's hair does have one curl out of place as they approach the center of the Earth. All of their hair gets wet and shaggy near the end of the movie.
Revealing mistake: Right before Alec nails the spike into the giant lizard's tongue, the blood bag he punctures is visible in the tongue.
Revealing mistake: When the group brings their raft out of the cave to the seashore, a dimitridon (the sail-finned dinosaur) attacks the group. Hans throws a spear into its mouth, and it dies, allowing the group to run the raft out to sea. We see several other dimitridons feasting on the dead one on one side of the screen, and the group sliding the raft to the sea on the other side. The sand on one side of the screen is a drastically different color than the other, one color ending abruptly and the other starting just as abruptly. That's 50's matting effects for you.
Plot hole: They all drop their backpacks and bags 3/4 mile down into the hole before climbing by rope into the caverns to begin their journey. Later in the movie, they are all sitting deep in the caverns complaining about the lack of water supply. Suddenly the long-haired guy pulls out a large glass bottle of gin, completely intact and they all drink from it. There is no way his pack could have fallen 3/4 mile onto a hard surface without that bottle breaking.
Continuity mistake: After the professor makes his speech at the end, he is talking to Carla. He puts his hand to his chin then puts it down. It cuts and his hand is up again.
Revealing mistake: When the group is rescued in the Mediterranean, you can see waves smacking against the matte background.
Continuity mistake: At the explosion in the lab, Jenny is blown on top of a desk with straight lines down the side. Nowhere prior or after shots is there any such desk. The next shot, Alec is picking her up from the floor.