King Kong (1933)

Revealing mistake: In the last third of the film after Kong has broken through the gate and is rampaging through the village, a native jumps out of his hut, loses his balance and falls by a chicken coop. His wig comes off and gets caught on the fence.

Revealing mistake: As the men walk past the downed stegosaurus, the camera moves faster than they do (judging from the background) but they stay at the center of the screen. Obviously the camera on the model of the stegosaurus moved faster than the camera on the men.

Revealing mistake: As one plane swoops at King Kong on the Empire State Building, another flies behind his head that is much larger than it should be relative to him.

Revealing mistake: The figures dropping from Kong's cliff into the river are obviously limp dummies.

Revealing mistake: When Kong breaks through the giant doors towards the islanders, part of the doors become transparent and you can see Kong's leg. This is a result of the matting special effects used.

Revealing mistake: When Kong crushes the scaffolding in the island village, it collapses not into a heap of splinters but in a neat panel of solidly bound wood.