Revealing mistake: When the boat is supposedly surrounded by a swarm of flies, they're really just relatively few flies trapped between two panes of glass in front of the camera lens. Evidence: whenever the camera moves, the "swarm" moves with it against the background, and you can even see some of the insects walking on the surface of the glass.
Revealing mistake: When the torpedo is fired at a Japanese ship, the underwater shot of the torpedo cruising reveals it being pulled by a long wire. It is also apparent that it was filmed in a tank with the reflection of the sun on the tank wall.
Revealing mistake: When Gort goes behind the wall to pick up Patricia Neal, the camera moves about an inch in the same shot, because they had to stop filming, go rig the wires up to Patricia, then start filming once again. (01:17:00)
Revealing mistake: After the Indians have attacked the wagon train Robert Taylor returns to see the devastation. When they call out the names of the dead we see a close up of each person and some of the "dead" are breathing.
Revealing mistake: Early in Act I, Kate Reid puts a glass in the sink (below our sight line) and turns on the faucet. No water comes out, but she reaches down and produces a full glass of water.
Revealing mistake: When the Christians are being fed to the lions, we can see that when a lion "attacks" one of the men, the man is actually smiling and visibly playing with that wild animal.
Revealing mistake: When Captain Quincy Wyatt interrogates a captive Seminole Native American by threatening him with a poisonous snake, the native is safe because there is a protective piece of glass between him and the snake. The snake's reflection is quite obvious on the glass panel. (01:14:40)
Revealing mistake: During the first shot of the rocket ship coasting through space stars can be seen through it.