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.
Revealing mistake: Clark uses his heat vision to warm up Alicia's cup of hot chocolate. When the drink is hot enough to start giving off steam, the "steam" is not water vapor. Water vapor would be nearly clear and rise straight up from the cup. This "steam" is more likely from a piece of dry ice as the vapor is rolling over the side.
Revealing mistake: As the helicopter that Lana is in dodges the falling meteors in mid-air, the camera shot focuses in on the cockpit. If you watch Lana's position as she first starts to appear she is semi-transparent and the frame of the window behind her is visible through her body.