Continuity mistake: When Karen struggles to keep the door shut, as Chucky tries to pull it open, Chucky stabs his knife through the door, cutting Karen's hand. Between shots the hole made by the knife gets mysteriously longer, and all the holes Chucky makes in the door seem to change shape/size/location. Also, when Chucky stabs through the door, cutting Karen, a small stain of blood is left on the door. Between three consecutive shots the blood is there, then disappears then reappears.
Picture added by Hamster