After Laura gets shocked during the storm, she and Jason jump around excited about their experiment and the kite string is no where to be seen.
When Jason explains in school how the talking machine works both doors to the school-building are open. When Laura runs off both doors are closed.
On at least two occasions the word "airplane" is used in the dialogue. Airplane didn't enter the lexicon until the 1930's. The original word was aeroplane and it wasn't used until after the Wright brothers flights. It was Winston Churchill who eventually initiated the change to airplane. Pre-20th Century people would most likely have referred to a "flying machine."
During the storm, Jason tells Laura the umbrella is important because it will keep the ribbon dry. However, multiple times the ribbon is shown to be soaked from the rain.