Clark picks up a haircut and tan between when we saw him rushing to Lana's help at the last episode and the beginning of this episode when he gets to the truck up in the tornado (due to the season break). For those of us with short memories, the opening clips make this difference obvious.

Red

When Jonathan Kent goes to speak with Clark at his school, Clark shoves his father into the driver-side door using his powers, thus causing a large dent in the door. If you look carefully just as Jonathan initially exits the truck, you will see the dent is there before Jonathan meets it the hard way.

Nocturne

The helicopter should have pulled Byron off the ground, not the other way around. He got stronger, not heavier.

Redux

You can see those light reflector things in the reflection of Lex's sunglasses when he's talking to Principal Reynolds. They're the huge white squares.

Note that Clark has lost his tan since last week's episode... because of course, this episode was filmed last year when Tom Welling didn't have a tan.

Lineage

The phone number for LexCorp on Lionel's business card has a 121 area code. Currently there are no area codes in the U.S. that start with 1. "212" is New York, while Kansas has a variety of them starting with 3, 6, 7, and 9.

Skinwalker

In one of Lex Luthor's last scenes where he's talking with Lionel they show his sunglasses on and then from the side they're off.

Visage

In the pool scene, Clark and Lex are playing pool, but the pool balls changes throughout the scene.
[Thanks Laura for the info]

Insurgence

Unintended Hidden Mic?: When Clark and Lex are seen talking in Lex's office, right before Lex finds the camera in his stereo, a shadow of a sound recording mic can be seen above them, on the wall.

While Clark is speeding/flying over the street between Luthor's and Daily Planet, the camera films from above clark in slow motion, but still there's one white car in the street which goes at normal speed.
[Thanks Thomas R. for the info]

Rush

When Pete punches Clark with the kryptonite outside the Talon, he goes back and crashes into a blue Daily Planet newspaper stand, where it becomes dented and falls to the ground. But when Lana throws the water onto him in the next scene, the newspaper stand are not present.
[Thanks Laura for the info]

Fever

Clark has a zit on his chin. He has invulnerable skin - how can he get little dirt particles in his pores?

In the beginning, when Martha coughs and drops the pitcher breaking it into a sharp barb in the ground. She faints and is seen to be falling into it, but Clark catches her. When we see the long shot, the broken pitcher isn't there.

Rosetta

In the opening sequence you can pretty clear see the back-wire supporting Tom Welling in the air in the first shot where you see his entire body from the side.

Precipice

When Lex is at the hospital after Helen got attacked, there is a blood stain on his shirt. During the train scene, which takes place right after it, that blood stain is gone.

The train car has window instructions on it in four languages, including French and Chinese. You're not likely to see that in a Kansas train station although you would in Vancouver, Canada (where the show is shot).

Accelerate

In the graveyard, the rain pours down and Clark is drenched. When he goes to the Talon, however, he is completely dry.

There's no air holes in the bunny box Lionel brings to Emily.

Exodus

Clark has a pimple on his chin again - something that seems a bit difficult for someone with near-invulnerable skin. (Look for it when he and Lana are out with the horses.)

The position of the horses radically changes position depending on whether the camera is doing a close-up or wide angle shot of Lana and Clark.