Revealing mistake: At the end, there is a group of people with their backs to the camera, facing & waving to the Fantastic 4. As the F4 fly towards the camera and out of the shot, the crowd turns to continue waving to them. One small boy in white remains facing the wrong way, still waving, until another child grabs the back of his clothes & turns him to face the right way.
Revealing mistake: In the beginning, when we see the sailor fall over the side of the boat onto the ice, watch the closeup first shot when he sits up - it's a (quite poor) green-screen shot with an image of icy water stuck in behind the actor. The ice has absolutely no sense of depth, the lighting on the background doesn't quite match the actor, and it doesn't match the following few wide-shots when the background is computer-generated.
Revealing mistake: When Johnny and Reed are talking about the bachelor party in the lab, they keep moving from one side of the table to the other: One side has a green book, the other a red one. Note the books behind Johnny and you'll notice by the colour and the reversed text that the shot's been flipped.