Deliberate mistake: Clark's CPR form is terrible. When he performs emergency breaths on Lex, Lex's chest doesn't rise and fall, indicating that he's not actually breathing much air in. He doesn't bother to measure for the proper location for chest compressions, a very important step that is the difference between proper CPR and simply cracking ribs. As he performs compressions, he is not chanting the rhythm (one-and-two-and-three) and his elbows are bent. All of these were altered for dramatic reasons or the purpose of safety, but the sloppy technique probably would have killed Lex.
Other mistake: Lois digs up Chloe's grave site to unearth her casket. It is obvious that the casket is not buried six feet deep. You would think, will all that is going on, that LuthorCorp would have at least buried the casket deeper to deter inquisitive eyes and maybe throw in a few scattered bones for good measure.
Visible crew/equipment: When Clark finds Lex in the Luthor mansion, they walk down the hall and into a new room. As they enter, as Lex is saying "So, hows the new ride?" if you look in the bottom left of the screen you can see a white tape marker on the floor, where Lex will walk over to subsequently.
Audio problem: Clark listens to his father and Earl talking in the hospital through the visitor window. The sound is the same volume and quality as when Earl was using the microphone, which he is not using now. Even if Clark used super-hearing to eavesdrop, it wouldn't sound the same as the microphone.
Revealing mistake: When Tina puts the necklace on Clark to disable him, he falls to the ground of his barn. As he is lying there, his collar is very dark, and his shoulder is lit up. Since the source of the green light is supposed to be the necklace, his collar should be the brightest lit.
Continuity mistake: When Lana shows Chloe the picture of her mother and Henry Small. We see that Chloe holds the picture on its bottom corners with both her hands. Cut to a close up of the picture but now she is holding it in the bottom middle of the picture. Cut back to the previous angle and she is holding it by the corners once again.
Continuity mistake: When Chloe is first asking Pete and Clark the questions for her poll you can see that the top page on her clipboard is yellow. Then she shows it to Lana and the camera gives you a nice big shot of the front piece of paper and it is white. When Chloe says "its unleashing the inner Lana." the front of the clipboard is shown again and now the paper is yellow.