Continuity mistake: Lois leaves the gas station but the reflection of the glass shows it still there.
Continuity mistake: At the heliport, a cork bulletin board on the wall appears and disappears between shots.
Continuity mistake: When Lois is about to ride on the helicopter, there's a man in the background easy noticeable because he holds two very bright glowing sticks. When the angle changes to a ground level view of Lois's legs, the man disappears.
Continuity mistake: On the Golden Gate, the school bus smashes a blue beetle car, a second later the car appears in perfect condition.
Continuity mistake: When the helicopter is spinning around two men run towards the control room. In the next angle they're gone, only to appear entering several seconds later.
Continuity mistake: The huge lamp post next to the bus on the Golden gate disappears when the bus is about to fall down.
Continuity mistake: At the end of the falling Hollywood sign, the girls are zooming away from the sign. Angle cuts brusquely and, though the sign keeps falling on top of them, the girls are now walking calmly forming a neat cue.
Continuity mistake: When the gas station collapses, at first the roof falls down completely and covers the gas tanks, but in the next shot it's slightly laying sideways.
Continuity mistake: When Hoover Dam is about to explode, there's a shot of a resort nearby. The people running close to the fallen umbrella differ between shots.
Continuity mistake: Superman lands on the sidewalk looking for Luthor and a group of people gather closely together behind him. In the next shot, half a second later, they are standing way apart from each other.
Continuity mistake: The blanket used by baby Kal El to get to earth is red, blue and gold. When he lands it's just red.
Continuity mistake: When Superman pulls Lois' car out of the quake hole, the camera then does some wide shots, rotating around the scene before Superman flies off. The damage to the front of the car changes in these shots.
Continuity mistake: After the van crashes against the beetle car, the new angle shows the van in a different position.
Continuity mistake: After Superman saves the train from derailing, there's a wide shot of the lead locomotive and it has just one small window on the side. In the immediate close-up there's a big window where the conductor peeps, and a sudden second window with a handle bar where another engineer shows up.
Continuity mistake: When the cab arrives in front of the Daily Planet, the sidewalk in front of the office is empty. A split second later there are people everywhere.
Continuity mistake: When Lois interviews Clark, the position of the glasses changes between shots.
Continuity mistake: When the cradle takes off, thousands of glasses fall on the floor, but the next travelling shot shows just four or five around the parents, and the rest of the floor clean.
Continuity mistake: When Superman arrives at the dam, after chunks of concrete have exploded everywhere seconds before, there is barely nothing broken, except for some cracks on the wall.
Continuity mistake: The cheerleading equipment is knocked down from the bench, but magically appears up on the next shot.
Continuity mistake: The Hoover Dam breaks in many pieces with huge cracks everywhere. When Jimmy is hanging about to fall, from his POV there are just four tiny cracks.
Answer: He wanted to see for himself if the stories were true. Some reporters tend to exaggerate the facts and if he had any other weaknesses. He couldn't be sure the kryptonite would work.
While I don't exactly disagree that Lex wanted to see for himself how invincible Superman is, I don't think that's the main reason why he did it. The bullets and fire were simply a charade to make it look like this was his security system. Keep in mind, he wanted Superman to enter his lair because the real trap was the Kryptonite that he had in the lead box.
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