Factual error: The six-wheel plastic G. I. Joe van that occurs beside some other toys in the Carvers' house was manufactured by Hasbro only in 1983. The film however takes place in 1973.
Factual error: The foliage in Paul's hometown is shown incorrectly in one scene. We are told that this is a suburb of New York City. We also know that it is around Thanksgiving, a time of year when the trees have shed their leaves for the winter. During the scene where Ben drives Paul home from the train station, all the trees in the background have their leaves.
Factual error: There are many errors in terms of location in the town, but you probably wouldn't notice unless you happen to know the town of New Canaan, CT. For instance, the sequence when Wendy is riding her bike makes no sense because the shots of where she rides down the main street are out of order. Also, as Wendy is talking to Sandy while on her bike you see a toy store in the background. That is actually a sports store that they covered up to look like a toy store.