When Lizzie attempts to confess to Kate in the hallway she is wearing a red and white shirt with no bag. However in one shot from behind, you can see that there is a blue bag strap over her left shoulder. This is the same blue bag she can be seen wearing in later scenes.
When Lizzie and Gordo are sitting at lunch, Gordo is sitting there playing with his fork. But then when Miranda comes over to sit down with them, Gordo now has his arm down on the table and is no longer playing with his fork.
Kate opens her locker to find it stuffed with toilet paper, and the toilet paper wall collapses on her. However, we can see a 4x6 actually pushing the toilet paper from inside the locker onto Kate.
An "X" on the ground is visible when Lizzie is talking to Kate before Kate does her routine.
When Lizzie enters the gymnasium to confront Kate about what she sent via the instant message, as she stops in front of Kate you can see the nice, big red tape mark at her feet.
After the McGuire's dig back up Lizzie the lizard; as they walk across the garden (with Mrs. McGuire telling Matt that he should open the box) if you look from the point where they began to walk from, you can see a white tape marker on the ground.
When Lizzie and Gordo are sitting at lunch, Gordo says that he doesn't like broccoli (referring to the pile on his plate) and that he tries to avoid it. However, it is established in many scenes that Lizzie's school has a self serve cafeteria, so why does he have broccoli?