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.
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.
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.
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.
xx:xx:xxRooster of Doom
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.