Continuity mistake: When Lois dies, the first time Superman sees her the left side of her face has sand and rocks around her cheek. When the angle changes they are at shoulder level.
Continuity mistake: When Superman avoids the train from derailing, the wide shot shows his cape fluttering but the close one not.
Continuity mistake: When Superman moves the school bus away from the void, the wide shot shows a different fence and wreckage, with a long piece of fence missing. The close shot shows a missing space that is slightly bigger than the bus.
Continuity mistake: When Superman saves the wrecked plane, from the outside shots the window has droplets of water, but from the copilot's POV the glass is clean.
Continuity mistake: The way the word Otisburg is written on Lex's map changes between shots.
Continuity mistake: When Ms. Teschmaker is watching the Metropolis news, the glass by the TV swaps from being behind the sun-lotion cream to in front of it.
Continuity mistake: When Luthor takes the kryptonite out of the box, from a side view he holds it at waist level, from a front shot he holds it at chest level.
Continuity mistake: In the wheat field, the clothes inside the cocoon change from being tucked inside a corner to visible.
Continuity mistake: By the craddle, Jor-El's hands are appart, a frame later they're holding the crystal.
Continuity mistake: When baby Clark is greeting the Kents inside the cocoon, Martha's hands change from resting on the sides to up.
Continuity mistake: In the footbal field, one of the megaphones that Clark kicks down appears on the bench in the following shot.
Continuity mistake: When the truck falls on Jonathan Kent, the way Martha holds him changes between shots.
Continuity mistake: When the player takes Susie with him, a bucket appears from nowhere on the right side of the bench.
Continuity mistake: When the football game ends, Clark is holding a bundle of towels. A frame later, the towel is neatly folded.
Continuity mistake: Before Superman enters Luthor's lair, Teschmacher is laying on the couch and a bottle of Ambre Solaire is facing straight close to the TV. In the next shot the bottle is turned to the left.
Continuity mistake: After Lex Luthor opens the box with Kryptonite, Superman leans with both arms on a table. A second later he's leaning with just one arm.
Continuity mistake: The first day Clark Kent works in the Daily Planet, he sits by his desk and then a close-up shows Lois placing her hair behind her ear. In the next shot the hair has changed and is now is covering her ear.
Continuity mistake: The day young Clark Kent leaves his parents' home, there's a shot where mother is watching him from inside the house. The clouds and sky colour reflected on the glass is different to the one we see in the following angle.
Continuity mistake: After Jor-El places the green crystal on Khal-El pod, it keeps changing place.
Continuity mistake: When Superman saves Lois from the car crash, the bumper's damage changes between shots: sometimes it's dangling on the edge, others it's attached to the car.
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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