Factual error: In the very beginning of the movie broadcasters around the world are commenting on the mysterious spaceship. To lend a bit of calm and normalcy one broadcaster notes that despite what is happening many people are enjoying the fine spring weather in the Capital, to this end picnics and the wonderful cherry blossoms are plainly depicted. Shortly afterward when Klatu leaves the hospital, he is seen examining the cleaner's tag on the suit he "borrowed". This is where he adopted the name Carpenter. Clearly printed at the top of this tag is the date 7/18 1951, not spring by at least a few months.
Factual error: When Bobby and Klaatu are leaving the Lincoln Memorial, Klaatu asks Bobby where Professor Barnhardt lives. Bobby responds, "near where my Mom works, at the Department of Commerce." There is no residential neighborhood as depicted as the location of Barnhardt's house anywhere near the Department of Commerce.