Continuity mistake: When Clark and Lex were talking about Helena while playing billiards, Clark picks up the stripped blue ball. They then continue to talk as he plays with the ball in his hand. He then puts it back on the table allowing it to roll but now its has changed into the cue (white) ball and the stripped blue ball can be see in the foreground right next to the side pocket and a stripped red ball.
Continuity mistake: Lex and Clark are playing pool. Lex misses his shot on the 12 ball and scratches which leaves the 12 ball near the pocket. After Clark gets the cue ball, he makes his shot on a different ball and the 12 ball that was next to the pocket is gone without it being knocked into the pocket.
Continuity mistake: After Clark sees Whitney and realizes that he may be Tina Greer the next scene is at the school newspaper The Torch. The first shot of the computer, as the camera pans past it, has a story about Whitney's return up front with the story about Tina Greer's "suicide" behind it. Even though nobody touches the computer in the time between shots the next shot of the screen has just the story on Tina Greer and Whitney's story is completely gone.