Plot hole: In the episode Onyx, Lex's alter ego holds him hostage in the wine cellar claiming that when they were kids, Lex accidentally got locked in down there. In the pilot episode, it is established that although Lionel had the castle shipped over from Scotland brick by brick and built, they never moved into it.
Continuity mistake: As the computer says "Subject seeds exposed to radiation" there is a camera shot showing the temperature monitor in the lower right of the shot. In that shot the readout shows the temperature reading is close to 100% of maximum. Then, in the next shot as the scientist says ".absorption level: as expected" the temperature monitor is now close to 90% as we see it climb up to the 98% they were trying for.
Continuity mistake: Clark goes to see Lex about the dog. Lex, who was about to start a game of pool, breaks a fresh rack. Lex then goes to make his second shot from near the side pocket and as he does you can see there are two striped balls near the rail close to Lex's cue stick. Just after he takes his shot those balls are gone even though those balls had not been hit.
Revealing mistake: After Lex's employee walks out of the office, Lex's gets mad and throws most of the objects off his desk. Prior to this, we a computer screen displaying something. When the objects go flying off and hit the wall, we see that it wasn't actually a computer screen but a prop that was made look like one. It was just the frame, a piece of paper that served as the display covered by clear plastic to make it look like an actual computer screen.
Continuity mistake: When Pete and Jonathan corner Clark in the cornfield, Pete opens a lead box containing green kryptonite to weaken Clark. The position of how Pete holds the lead box changes every cut. At first he is holding it with just his right hand, then just his left, then both his hands.
Continuity mistake: When Chloe tells Pete about her discovery about the Red meteor rocks, she gives a sample to Pete who then looks at it. At this time the red meteor embedded in the rock is above Pete's left index and middle fingers. Cut to a close up of Pete's ring and the rock but now the rock is embedded in between Pete's left index finger and thumb.
Continuity mistake: As Chloe leaves Clark calls out her name. In the shot from Chloe's side of the room, Clark is holding the pitchfork horizontally low in front of him. The shot switches, as Chloe finishes her turn, and now Clark is holding the pitchfork with his arm bent so that the pitchfork is no longer low in front of his waist.
Revealing mistake: After everything with Dawn is over Clark goes back to the prom. Clark asks Lois if she wants to dance. Lois says "I'm not the one you want to dance with." and she nods to the door. The shot switches to show Lana walking in to the prom. When Lois makes her nod to the door Lana would have been completely out of sight. Lois nods before Lana even enters the room.
Continuity mistake: Alicia's hair keeps changing as Clark holds her in his arms. In some shots the hair on her right side is messy. When the shot switches to a close up, her hair becomes neatly combed and tucked behind her ear. All of this occurs after Clark burns the rope she was hanging by.
Continuity mistake: At the start of the final race there is a bridge at the far end of the road that Pete and Jason are going to race to. During the entire race they never pass this bridge yet it ends up behind Jason's car when Pete looks in his rearview mirror as the crash is going on.
Visible crew/equipment: As Clark is ambushed inside Belle Reve, in shots focusing on Lex as he is being beaten by the twin and again a few seconds later, there is a dark object in the upper right corner of the shot. There is nothing in this scene to explain it and it moves with the camera so it is probably some piece of filming equipment. Regardless of what it is, it should not be in the shot.
Continuity mistake: As Lana starts her second swing at Chloe with the axe while the camera is in front of Lana Chloe is just barely getting up and is still close to the floor. The camera switches in mid-swing and suddenly Chloe is completely standing and is able to jump back to dodge the swing.