Revealing mistake: Most of the posed, fake displays in the ape's Museum of Natural History contain real people who are slightly moving if one pays careful attention. Especially noticeable are those posing with objects held in - or above - their hands.
Revealing mistake: Toward the end of the film, in a long-shot, an ape hits another in the head with the butt of his rifle, knocking him over. You don't even have to look closely to see that the rifle's butt never even comes close to making contact with him.
Revealing mistake: Towards the end of the movie, at the cave, you can see that the rifle Charlton Heston has been firing has no magazine inserted.
Revealing mistake: Throughout the movie, if you look closely at all of the apes' mouths, you can see the actors' teeth behind the ape teeth.
Revealing mistake: If you look closely at the back of the head of both Zira and Dr. Zaius, you can see the actor's real hair.
Revealing mistake: As Lucius tackles the guard and yells "Grab him!" to Taylor, Lucius then strikes the guard over the head, knocking him unconscious. When Lucius begins pushing the guard over to Taylor's cell, you can see the guard is already unconscious before he gets struck. (01:18:45)
Revealing mistake: When Taylor approaches Miss Liberty, there should be horse tracks in the sand extending to the end of the frame. About 1/3 of the frame shows smooth sand preceding some horse tracks.
Revealing mistake: During the trial Honorius walks around the room and stops to deliver his words. Watch his feet and you'll notice a dark spot right where his feet are, which serves as the marker.
Revealing mistake: After Taylor gets up from being dragged in the square, another ape tries to rope him. As this occurs, the stuntman is easily distinguished.
Revealing mistake: During the desert walk you can see footprints in the sand from previous takes, also dark lines in front of them which they follow like a pathway.
Revealing mistake: In the scene where Taylor is running after the humans when their clothes are stolen, they're supposedly naked. As Taylor is running through the trees, you can see he is wearing flesh coloured shorts. (00:28:45)
Chosen answer: Dr. Zaius did know the true history of man and ape, but he deliberately hid the truth from the other apes. For Zaius (and other high-ranking apes who were guarding the secret), it would be shameful and demoralizing to ever admit that humans were far superior to apes in the past and that they could, potentially, conquer the apes. In more than one scene (such as the paper airplane scene in the first film), we see Zaius obviously frustrated that Taylor's very existence threatens to expose the truth.
Charles Austin Miller