At the hospital, when Clark is looking at his reflection, the blanket on his back isn't covering his left shoulder. When he moves away, and the camera changes, the blanket fully covers both his shoulders.
When Lana traces the likeness of Countess Margaret Isobel Thoreaux, she clearly starts on the left chest area, yet the first view of Lana's work is that of the Countess' face.
Jason says the next day is Lana's birthday, but the first season episode in which her birthday was celebrated didn't air until the end of November, well into the school year.
When Lana takes a shower, the steam is rising before she turns the water on.
Chloe's gravestone clearly lists her birth year as 1987. However, she turns 18 in "Spell", indicating that either the gravestone is a production error, or "Spell" actually takes place several weeks after its November 2004 air date, in early 2005, or perhaps those in charge of faking her death were unconcerned with accuracy.