Visible crew/equipment: When Lex and Chloe are talking after the morgue, you can see a crewmember passing behind her at 30:53. (00:30:53)
Suggested correction: You see a person walk behind her in a corridor. Since it is very obvious and occurs during a short cut where nothing else is happening, it is deliberate. A fundamental rule on film sets is that once "action" is called, no-one moves if they don't have to. If a crew member had walked through the shot like that, they would have done a new take or just use a different segment of the shot.
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, with 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.
Other mistake: In the opening credits of the original broadcast, Kristin Kreuk's first name is misspelled as "Kristen." The credits sequence, which was new for Season 5, was corrected by the time the second episode of the season aired.
Continuity mistake: When Jonathan lectures Clark about the danger of accepting gifts from Luthors, the key he twirls is a GM truck key, not a Ford truck key.
Continuity mistake: After the meteor shower, the bit of hair that Mr. Luthor finds in the cornfield changes in his hands from stretched straight to curled. (00:06:15)
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. (00:33:10)
Revealing mistake: There are times when it is quite evident that Michael Rosenbaum is voluntarily bald.
Factual error: Lex crashes and his car lands on its roof, but in the span of a few seconds it has sunk straight to the bottom, and turned the right way up. Cars don't sink that quickly. (00:17:25)
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.
Continuity mistake: The hole in the guard rail changes size and shape between the shot of Lex driving off it and another looking up at it from below. (00:18:10)
Continuity mistake: Season 7, episode "Persona". When Lana walks in from the balcony her hair swaps from being in front of her shoulder to behind it, then back again. Then when she sees Bizzaro's face in the reflection of the glass, her hair is styled differently again.
Visible crew/equipment: When Lex spins up to Luthor Corp in his car, as the camera pans over the car to follow him, the cameraman is reflected in the window as he pans over. (00:14:40)
Continuity mistake: In the truck right before the parade passes them by, Jonathan Kent kisses his wife. Cut to a different angle but this time his left hand is on her cheek. It wasn't there in the prior angle. (00:02:15)
Continuity mistake: After Greg hits Clark with the pipe in the foundry, Clark's hands are very dirty. In the closeup on Clark's hand immediately after he lands, the hand is clean to show the effects of the Kryptonite. (00:39:45)
Revealing mistake: While there was a close up on Clark while he was asleep, you could see a small cut right above his eye, which is impossible if he can't be hurt.