The Lost World (1960)

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.

Trivia: In the movie, the native Indians were the bad guys, but in the book, they were the good guys. They were fighting the ape-like species living there, and were helped by the group from England. They however did try to keep the group from leaving the plateau, wanting them to stay.