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)
Revealing mistake: Pat Boone falls out of a tree naked. But he is wearing skin-colored foot thongs on his feet so he won't cut himself while walking away from the fallen tree branches that were holding him. (02:04:50)
Revealing mistake: A giant lizard is stepped on by Boone. The lizard's skin turns red after being stepped on. It becomes so red that a nearby rock begins reflecting the red light. (01:55:40)
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.
Revealing mistake: When evacuating the flood through the broken stalactite hole, Madam and Mason are standing on some sort of platform to get through the hole.
Revealing mistake: Arlene Dahl hangs on to a stalactite as the water rises in a cave. At the ceiling you can see it swaying and breaking apart but the portion the woman holds doesn't move at all. (01:10:40)
Revealing mistake: When the group is rescued in the Mediterranean, you can see waves smacking against the matte background.
Revealing mistake: When the four travelers are riding up the volcano in the asbestos basin, they are obviously dummies.
Revealing mistake: Pat Boone gets lost. He walks over to a rock bridge, over a phosphorescent lake, which breaks up just before he steps on it. The larger chunk begins to fall but stops and bounces back up a bit because it hits a stage. (01:13:30)
Revealing mistake: As Mason's party rides a giant offering bowl (on magma) up a volcanic vent, you can see a straight seam joining the tubular vent on the bottom portion of the screen. (02:02:30)
Answer: It's just an added sound effect that foreshadows an ominous event (the giant reptile) that is about to happen. It's not meant to be anything naturally occurring.
raywest ★