Jan's wardrobe is mismatched during same day scenes, i.e. downstairs in one outfit then upstairs in another outfit.
They must have filmed two different 'days' on the same real day, but Eve Plumb (Jan) didn't change her clothes. When Jan is studying with Clark on the first day, she's wearing a long-sleeve white shirt with a blue striped design, dark blue pants, and has a red yarn ribbon in her hair. The next day, Mike and Carol greet Jan as she gets home from school, and ask how her day went; she's wearing the exact same outfit. She leaves them and walks upstairs. When she confronts Marcia in their room, she's wearing a totally different outfit: long-sleeve shirt with brown and green stripes, dark pants, and a green yarn ribbon in her hair.
Jan asks Clark to look over her picture of the United States, because it looks wrong. Clark responds that she forgot to add Baja California. Baja California is in Mexico.
When eating dinner, there's no food on Marcia's plate.