This seems to occur frequently. In Pearl Harbor, a pilot flies around the tall mountains of Long Island--or, at least, that's where the scene is set--before departing New York City from a train station that looks suspiciously like Union Station in Los Angeles. And in The Battle of the Bulge (1966), the Germans were using American tanks made after World War II.
I love the war movies where everyone is so clean, hair combed and well shaved.