Plot hole: Clark is possessed by Dawn. Jonathan shows up and he has a chunk of kryptonite. Jonathan hides the kryptonite behind his back and Clark's body does not feel it until Jonathan takes the kryptonite out from behind his back. What, is Jonathon made of lead or something?
Continuity mistake: Jonathan reveals the kryptonite to Clark/Dawn and Clark starts to react to it. Because of the kryptonite Clark drops the tiara he took from Chloe and it hits the floor near Chloe's hand. Then, just before Dawn is forced out of Clark's body, the tiara has slid several feet away and is now close to the furnace. Clark turns his attention to Chloe, she is faking an injury at this point, but now the tiara has moved back to be near her hand.
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: Jason accesses Lex's computer and Lex catches him. After a discussion on an email Jason found Lex says "Burying ladies in the backyard isn't exactly a hobby of mine, Jason." Lex then just stands there staring at Jason. During their stare down the camera jumps back behind Lex and then back to behind Jason. During these camera changes the lighting on the left side of Lex's face changes. When the camera is behind Jason, Lex's left side is dark. When the cameras is behind Lex, Lex's left side is lit up.
Plot hole: When Dawn enters Clark's body, she slaps Chloe in the face with the back of the hand, and Chloe goes flying. A few minutes later, we see Chloe get up and say that she's fine. Getting backhanded by Clark would make you very far from fine. First of all, Clark can knock someone out just by tapping them on the forehead, as seen in a previous episode. Second, Dawn has no idea of Clark's powers, so she would have used way more force than she actually needed to. The amount of force inflicted from the attack should have caused fatal injuries to Chloe. There's not even a cut or bruise or anything.