Continuity mistake: As Lana is walking up the stairs to Clark's barn loft, from the front shot Clark has his hands to his side. When the camera jumps to behind Clark, watching Lana ascend the stairs as she says "I, uh, came by to say thank you." now Clark's right hand is behind his back and not to his side.
Continuity mistake: Martha is putting together a flower arrangement. When Jonathan is talking about Clark not eating the last piece of cherry pie, in a shot of Martha and Jonathan, Martha is picking up a branch of white flowers and holding it in front of her. When the shot switches to Clark, you can see her putting this same branch of white flowers in the vase even though she made no motion to do so in the previous shot. This is supposed to be continuous since you can hear Jonathan talking between the shots.
Continuity mistake: When Seth is testing his powers in the hospital room he tries to move a cart at the foot of his bed. To do this he has to raise his hand to use the magnetic power he has. In shots from the front his right hand is palm down using his power but when the shot switches to behind Seth, to show the cart starting to move, his palm is now turned towards his left. His hand is back to being palm down when the shot switches back to the front.
Continuity mistake: Seth fires at Clark and Clark dives behind a ride car to dodge the bullets. As Clark stands up he brushes the impacted bullets off of his shoulder. Seth says "Looks like I'm not the only one with special powers." before he turns to use his powers to pick up a ride car that is behind him. As Seth starts to get the car in the air, in the background you can see Clark is only just standing up and brushing his shoulder off again.