Revealing mistake: When Sue Snell wakes from her nightmare, listen carefully as her mother comforts her. At one point, she calls her Amy, which is the actress's real name (they're mother and daughter in real life).
Suggested correction: Sue's mom just keeps repeating it's all right, you never hear any name mentioned.
That's because the music is supposed to drown out any dialogue.
Revealing mistake: When Kong first shows up to take Dwan away, he roars and pounds his chest. As he does so, the breastplate in Rick Baker's costume vibrates visibly in a way that a flesh-and-blood chest wouldn't.
Revealing mistake: Navarro is climbing on the outside of the train, the overhead shot shows very wide gaps between the plates welded to the windows. When the shot changes to a side-on view, the gaps are considerably smaller. This is seen several times throughout the quick scene changes. (01:47:00 - 01:49:30)
Revealing mistake: When Damian knocks Lee Remick over the balcony with his tricycle, five big goldfish in a bowl get knocked over, too. But the fish don't move; they don't flap around like fish do out of water. They are obviously fake fish.
Revealing mistake: When the armed gang abduct the Mayor, a LAW rocket is fired at a Lorry, the few seconds are obviously several shots pieced together. The giveaway being the driver is a dummy as the driver has no face, even after the explosion, the faceless dummy just falls out of the cab.
Revealing mistake: When Babe is hiding in the bathroom and his attackers are prising the door off the hinges, you can see the wall move as well as the door, so showing that it's a set rather than an apartment.
Revealing mistake: By order of the shrewd Harry Webb, the helicopter just destroyed the Cessna on the ground. The two guys decide that they are gonna walk back to civilization. Cut to the vehicle driving to the explosion site, and you can see in the desert sand ahead of the jeep fresh tire tracks from previous takes. And it keeps happening during the desert chase. (00:10:20)
Revealing mistake: In the second murder, the killer of the first one is bashed in the head by a monkey wrench, her head split open. A second hit ensues, but there's no reaction to it, no further spraying or damage, the head itself barely moves, and with delay. (00:04:30)