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.

Challenger: Ah, Professor George H. Challenger and his lost world! Have you got your camera, Malone? I think a photo is in order.
Malone: You're quite right, Professor.
[Snaps a picture of Jennifer Holmes.]
Challenger: Reporters!

Trivia: Thanks to the money guzzling Cleopatra, the film company that put it out was forced to cut back their budget for other films like this one. Irwin Allen (director of this B grade epic) was forced to use real reptiles as dinosaurs instead of stop motion.