Maria von Trapp's maiden name was Kutschera, not Gustl. Gustl was a nickname for her middle name, Augusta.
The "Ave Maria" that the family sings in concert was composed around 1970 by Vladimir Vavilov, so during the early years of World War II it didn't yet exist.
Maria pronounces the Captain's name as "George". The name is spelled "Georg" and pronounced "GAY-org" regardless of the language of the film.
Young Agathe has dark brown eyes; the older Agathe has blue eyes.