Revealing mistake: When Marion is driving her car through the night, she keeps looking at her speedometer. You can clearly see that the gear shift lever is in "park" as she is driving.
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: When Crockett, Bowie and their men are about to destroy the giant cannon, several of the men are standing right next to it at the moment one of them tosses the torch on the touch hole. Then, there's a big flash at the touchhole. In the very next shot, right before the cannon explodes, we see the cannon, but everyone has disappeared. There was no time for them to flee completely out of sight.
Revealing mistake: When Henderson and his wife are walking on the upper deck before jumping off, you can not see any of the superstructure above them that should be visible.
Revealing mistake: During the initial breakout of the Gladiatorial training school an armed guard is stabbed on a balcony and falls to the arena below. He gets up and walks into the doorway adjacent to where he had fallen, apparently satisfied that his part was completed.
Revealing mistake: Midway through when Ian Carmichael and his girlfriend are dining at a restaurant with Terry Thomas, they are talking and looking at the menu. The table is laid with cutlery and wine glasses - ready to eat. During their conversation there is a shot of Ian Carmichael only talking and smoking but there is no cutlery and the wine glasses have been replaced by a brandy snifter, obviously after the meal. The next shot shows the whole table with the correct pre-meal setting. When they have finished the meal, the table setting is as it was in the earlier shot.
Revealing mistake: When the prison van drives through the tunnel, then stops and reverses so the crooks can set up the road works, it's supposed to be driven by Wilfrid Hyde White. But it's obviously being driven by someone else, as the driver looks nothing like him.
Revealing mistake: If you look closely, you can see the strings moving the Ouija board planchette while it's floating in the air towards Medea. (It's easier to spot them on the high-definition Blu-Ray version, but they can be faintly seen in other versions of the film if you know where to look).