Continuity mistake: After Clark saves Lex from the ring of fire we see the area that was burned by the fire. The candle that Shannon used to light the alcohol is no longer on the floor. Since the candle was in the fire it should be partially melted and there would be some evidence of it.
Continuity mistake: As Alicia holds the kryptonite on Clark her hand turns between shots. While she says ".but you broke the rules first." there is a camera change. Before the change the frontal view of Alicia has her holding the kryptonite with the glowing part straight up. After the camera change she is now holding it so it points to her left.
Continuity mistake: The position of Clark's sleeve to the tourniquet change between shots. When Dr. Bryce first ties the tourniquet she does so at about an inch from the end of the sleeve. In the next shot, as Clark lies unconscious and nobody touches his sleeve, the tourniquet is tied around the sleeve and no longer on his skin. Just before Dr. Bryce sticks the needle in it's back to being an inch from his sleeve.
Continuity mistake: In Episode 22- Season 5 - "Vessel": At the start, Lex wakes up and heads towards an open field many miles away from the mansion. They show a camera shot of his face in the field and the sky is grey and cloudy, but in the very next shot the sky has turned blue and the sun is shining.
Visible crew/equipment: Episode 22, Season 5 - "Vessel": Lex returns to the mansion, after being abducted, with his new powers. When he shows Lana by shooting his hand, they start talking by the stained glass window. The reflections in their eyes are supposed to be the light coming through the window, but are actually from a white screen used for lighting. The reflections in their eyes are rectangular to confirm this.
Plot hole: In episode 4-16: "Lucy," there is no reason for the criminals to be chasing Lucy on the ski slopes at the beginning of the episode. We learn later that she's been working with them all along; she calls Lois during the chase, and the call is part of Lucy's plan with the criminals. Lucy being chased by the criminals would make sense given the story she gives Clark during the next part of the episode, but it doesn't make sense for what we ultimately learn is the truth. And nobody from Smallville is anywhere near the ski chase, so it also doesn't make sense as any kind of a ruse.
Character mistake: While Clark and Chloe are at Daily Planet, Clark asks her what is wrong with her mother. Chloe responds that her mother left when she was twelve years old. In the episode Lineage, Chloe and Clark are looking for Rachael Dunleavy and Chloe says her mother left when she was five.