Factual error: The aliens request a meeting with Dr. Marvin on the shores of the Chesapeake Bay, near Washington DC. However, the shot of him driving along the shore line shows mountains in the background, which don't exist in Eastern Maryland. (00:35:13)
Factual error: During the balloon ride the Basilica of the Sacred Heart of Paris is seen. It began to be built in 1875, 3 years after the date the movie is set.