Continuity mistake: When Lynette is at the school, the teacher is waiting outside eating. If you look at her hands, you can see that she has a white napkin. The napkin changes from one hand to the other between shots, and in one shot it is small and the other it is not.
Continuity mistake: At one point in episode 1.2 ("Ah, But Underneath"), Susan looks at the note used to blackmail Mary Alice and we can see that it has been folded twice and become somewhat crumpled. Two episodes later in "Who's That Woman?" (1.4) when the girls show the note to Paul, it is almost perfectly smooth with a single horizontal crease.