Factual error: At the end, after Ellie testifies before Congress, she departs the Capitol building to a waiting limo. The media is there waiting for her. Behind the reporters appears to be the Reflecting Pool, and the Washington Monument at the far end of it. However, in reality, the Reflecting Pool is actually located between the Lincoln Memorial and the Washington Monument, not the Capitol. While there are two Reflecting Pools, the columns in the movie are the columns of the Lincoln Memorial, not the Capitol. Furthermore, the steps leading to a street in front of the reflecting pool are located at the Lincoln Memorial; the Capitol has a large green lawn between it and the street.
Factual error: When Foster is going to Japan, a Harrier jet transports her to a ship. The ship is underway, traveling at quite a clip according to the wake. The Harrier landed on its heliport platform, facing the stern. Harriers hover, but cannot fly backwards at more than a crawl. It would have crashed.
Factual error: The last time Eleanor Arroway talks to S.R. Haddon, he's aboard the Russian space station Mir, and Haddon explains that he's up there because the "low oxygen" and zero gravity counteracts his cancer. In fact, there is no "low oxygen" environment aboard space stations or other spacecraft. Low oxygen content would, of course, kill any astronauts or cosmonauts in short order. The breathable air in spacecraft always has at least the same oxygen content as Earth atmosphere at sea-level. In fact, most Russian missions used excessive amounts of oxygen. S.R. Haddon's original dialogue was probably "high oxygen and low gravity," but the line was bungled and allowed to remain in the film.