Continuity mistake: When the Iron Giant follows Hogarth home they cross a railwayline and the Giant starts to eat the steel rail. During this scene you can see the distance of the railway sleepers changing from a fair distance apart to close together over the course of the shots. In the same scene you can also see that, although there is a crossing with lights, no vehicle could possibly pass as the rail has no crossover facility at that level. (00:24:30)
Continuity mistake: Kent Mansley, the investigating government officer, can be seen sitting (humiliated) in the back of an army truck in a convoy leaving the scrapyard (after he has been told by the general that 'he will be in charge of the toilets from now on'). A few minutes later he is driving a 'civilian' car in the middle of that same convoy. (01:01:05 - 01:04:45)
Continuity mistake: Throughout the film the size of the Iron Giant varies from being able to hide in a barn shed to sticking out well above a town to be spotted from a fair distance. A good example is when the Iron Giant launches himself after a fall from a cliff in front of Hogarth's mother.
Factual error: In the scene where the Iron Giant is spinning the boy around in a junk car he is holding, notice that the car resembles a 1959 Cadillac...however since the film is set around 1957, this is not a likely car to be found anywhere yet, much less a junk yard. (00:46:20)
Continuity mistake: When the Iron Giant first learns to talk, he picks up a huge boulder that wasn't there earlier. The boulder was in a pretty big hole, but when Hogarth walks away the hole has disappeared.
Continuity mistake: When the Giant is being electrocuted at the power station, he is on his knees, struggling with the wires. After Hogarth cuts the power, he falls slowly backward. But there's a cut right before he actually hits the ground, and suddenly his feet are out in front of him and not folded underneath anymore. (00:12:45)
Continuity mistake: When Hogarth runs away from the Iron Giant, he crashes into a branch and falls to the ground. Yet in the next shot when he lies down on the ground, there is no tree near him.
Continuity mistake: When the Iron Giant presses down the railroad, it still is slightly bent. Yet later on it is perfectly straight.
Continuity mistake: When Hogarth knocks his face into the tree after running, he has a lot of blood on his face, but in the very next shot, in which he turns around after hearing the giant, the blood is gone.
Continuity mistake: When the Iron Giant and Howgarth meet for the second time, they both sit down, and you can see that both the Giants hands are flat on the floor, then the Giant produces the electricity shut off switch from one of his hands even though both his hands were previously empty.