Revealing mistake: During the big underground fight scene, the solid rock wall moves as George pounds a Morlock's head against it.
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: During the Mutants' escape, while they're attacking others in the Citadel some of their rubber bald caps come loose, and we can see the caps' edges over their hair/heads.