The story takes place in 1949, but the aircraft that returns Mark to Hong Kong is Pan American World Airways N6535C, named "Clipper Mercury". It was a Douglas DC-6B, which entered service in 1952 and left service in 1961.
When Mark is at war writing and a butterfly lands on his typewriter, the close-up of the page clearly shows he was writing to inform her that her "twenty-third letter has arrived". In a scene not long after, the letter has reached her and as she reads, his voice-over differs, saying her "eighth and ninth letters have arrived". A line about his bottom hurting from bouncing around in jeeps matches both letters.
When the orderly comes to get Dr. Han in the operating room at the beginning of the movie, one of the members of the film crew (a man wearing glasses) can be seen reflected in the window of the right-hand door to the O.R.
During the scene with Mark and Suyin in the airplane: after they land and the announcement says they can remove their seat belts, both actors do as they are supposed to and release their belts. Then Mark stands up but you can clearly see through the aircraft window that the scenery still shows them in a taxi movement.