Deliberate mistake: There are no lights ever to be found in the underground world and the ceiling is literally the surface's ground, but there is still a perfect level of lighting regardless.
Factual error: James Franciscus arrives at a New York City subway station called Queensborough Plaza. Queensborough Plaza is an above ground station. Queens Plaza is the name of an underground station in New York City. (They are located within walking distance of each other, but are definitely distinct stations). (00:40:30)
Revealing mistake: When Ursus is giving his speech in the Ape city, many of the Chimpanzees and Orang-utans are wearing incredibly fake-looking masks. (00:17:15 - 00:19:50)
Revealing mistake: When Brent is attempting to get the reins for the horses after escaping his wheeled cell, you can see a second set of reins going through the wooden holes to where a driver is hidden. When he gets the fallen reins, the second set is no longer there. (00:38:00)
Other mistake: As Taylor is dying, there is blood on his hand. When his hand lands on the bomb's detonator after he dies, his hand slides off, but no blood is shown on the detonator. (01:29:35)
Revealing mistake: When Brent is shot dead, the blood pack is visible on his chest, poking out from under his bandaging. (01:29:15)
Revealing mistake: When Ongaro is pushed up against the spikes on the cell bars, the spike against his neck is shown wobbling like rubber. (01:18:50)
Revealing mistake: When Nova says "Taylor" whilst Taylor and Brent are killing each other, Ongaro turns around and Taylor goes to punch him. Taylor's punch completely misses Ongaro, but Ongaro still reacts anyway. (01:18:45)
Revealing mistake: As Brent and Nova run away from the apes, they get caught in a net. You can see they are both wearing shoes, even though they are supposed to be barefoot. (00:31:45)
Continuity mistake: When one of the apes shoots the horse, Brent and Nova fall off the horse and land about six feet away from it. Shot cuts and they are suddenly only about two or three feet away from it. (00:31:00)
Factual error: Brent enters the underground area via Queensborough Plaza. He and Nova walk some distance through (sewers?) and emerge outside the main branch of the New York Public Library, located on 5th Avenue and 41st St. in Manhattan. I'll concede that that's possible, since we don't really know how much tunnel they walked through. (It's about 2 miles and includes crossing the East River.) Only moments later, they're in front of the New York Stock Exchange, which is at 20 Broad Street, easily four miles to the south. In the next shot they're walking past Radio City Music Hall, on 6th Avenue and 50th Street. That's roughly a half-mile NORTH of the Library. I know stuff got shook up in the nuclear blast, but that's a bit much... (00:40:30 - 00:51:00)
Continuity mistake: When the Apes are trying to break into the mutant's church, Mendez looks towards the Alpha/Omega bomb, and is wearing a cream-colored jumpsuit with a purple sash. When the Apes do break in, his sash has disappeared. (01:25:50)
Plot hole: Brent discovers that Taylor is on the planet and has a connection to Nova through the dog tags she is wearing. However, throughout the first movie, Taylor never had any dog tags on his person. Also, his ship was destroyed, and he only managed to retrieve a life raft and supplies. His belongings and clothing were taken from him when he was apprehended by the apes. Taylor should never have had any dog tags to give to Nova or for Brent to discover. (00:11:10)
Visible crew/equipment: When Brent is standing in the black crash-landing crater talking to Nova, two of his camera marks are visible behind him: They are made of white tape on the black ground. Seconds later in a wider shot they are gone. (00:10:30)
Other mistake: Near the beginning Taylor gives Nova his dog tags. Where did he have these during the whole first movie that they were never found, and why would he have hidden them?
Visible crew/equipment: A human riding a horse is knocked off into a pile of hay. When the rider lands hay falls off the pile, revealing an air bag. (00:32:44)
Continuity mistake: When Taylor and Brent fight in the cell, just before Taylor jumps on Brent, we can see nothing on the floor behind Brent. Then as Brent gets jumped on, some blood has appeared on the floor out of nowhere. (01:17:25)
Continuity mistake: Zira says "Have you a horse?", Brent says "Yeah, out in the scrub." In this shot, Brent's left arm is facing Zira. Next shot, Brent is suddenly sitting about 90° to the left and his left arm isn't facing Zira. (00:29:05)
Continuity mistake: When Zaius and the general are in the sauna, as Zaius says "Well, General. I only pray you know what you're doing" and the general says "How can you have any doubt, Dr. Zaius", Zaius' hands are on his lap. Next shot, his right hand is suddenly placed on his face. (00:22:00)
Continuity mistake: Taylor walks over to the stone wall and tries to hit it with the rifle. In the close-up, as he aims the rifle at the stone wall, you can see his left hand is wrapped around the butt of the rifle. Between this shot and the next shot, Taylor's hand switches which side of the rifle butt it is holding. (00:11:40)
Chosen answer: The mutated humans (the ones with the bomb) have telekinetic powers and projected the image into Taylor's mind, the same way they did with the fire with the apes.
Bishop73