Continuity mistake: Stewart's arms are crossed when other crew first sees her dead body, and the next shot of her corpse shows her arms straight at her sides. (00:07:31)
Continuity mistake: At the beach when Cornelius reads the scroll his fingers swap between being under the scrolls to being on top.
Continuity mistake: When Taylor discovers Stewart's corpse his hand is on his chest but a shot later it's by his side.
Continuity mistake: While during the trial Honorius says Zira and Galen experimented on Taylor his right-hand swaps between being by his side or resting on the chair.
Continuity mistake: When Julius beats Taylor he protects himself. The position of his hand changes between shots.
Continuity mistake: Zira visits the cages and asks Taylor if it still hurts. Taylor holds the iron bars with both hands parallel to each other. A shot later his right hand is positioned higher.
Continuity mistake: After Zaius warns Taylor not to go to the Forbidden Zone, Taylor turns the horse to face Cornelius. In the closeup after Zaius tells Taylor "You may not like what you find", he turns the horse around again.
Continuity mistake: After the apes have blown up the cave, Taylor and Nova are shown riding away on a horse. In a close-up, Taylor looks at Nova and places his hand on Nova's cheek, but in the reverse angle he suddenly doesn't have his hand on her cheek and he is now looking straight forward. (01:44:30)
Continuity mistake: After Lucius knocks out the guard, Lucius then ties the guard to the cell bars. In the shot where Lucius says "I'm Doctor Zira's nephew. This abduction was her idea", it can be noticed the fingers on the guard's right hand are resting freely. Then as Taylor says "Get me out of this", the fingers on the guard's right hand are almost clenched together. (01:19:00)
Continuity mistake: When Zira looks at the "My name is Taylor" on the notepad she says "Get me a collar and leash." The shot then cuts to Taylor in the cell. In this shot, he has a few strands of hay along his arm, but in the reverse angle, there's hardly any hay on along his arm at all. (00:45:35)
Continuity mistake: When Taylor, Landon and Dodge do water check, during the shot where Landon says "Thunder and lightning, no rain", Landon is looking down on the ground. In the next shot, as Landon finishes the line with "If we could just get a fix", he is suddenly looking upwards. (00:17:35)
Continuity mistake: When the three arrive at the planet Taylor asks for water and Dodge shows his canteen. Check the strap of his backpack changing between shots.
Continuity mistake: When the movie begins Taylor starts rolling up his left sleeve with his right hand. Shot changes and his hand is away and the sleeve is already rolled. Up.
Continuity mistake: When Lucius releases Taylor from his cell after knocking out Julius' the guard, his hands are shown to be tied above the horizontal lining along the cell. Then as Lucius says "We've got to move fast", the guard's hands are now tied below the horizontal lining along the cell. (01:19:05)
Continuity mistake: When Julius starts hitting Taylor in his cell, Taylor drops the notepad and pen he is holding and it lands about 4-5 inches away from him. A couple of shots later, the notepad is suddenly an inch away from Taylor without being touched. (00:45:10)
Answer: I think that this is meant to be a mystery. Taylor/Charlton Heston, an astronaut, leaves a world set somewhat in the future after 1968 (when the movie was made) but still recognisable to cinema-goers at the time, to travel through a "time vortex" to arrive in a world in a distant future, which has changed beyond recognition. Taylor meets the orangutan Zaius/Maurice Evans, and Zaius hints that he has some idea of what had happened, but Zaius' knowledge is either limited, or else Zaius is not going to tell Taylor (or his fellow apes) the full story. At the end of the movie Taylor discovers that, at some point between his leaving his own time and arriving in the "Planet Of The Apes", the world had been devastated by a nuclear war, but I think that the exact time, causes of, and course of this nuclear war are deliberately left as a mystery. Sometimes I think a bit of unresolved mystery actually improves a story, and I think this is the case here.
Rob Halliday