HARRY POTTER AND THE SORCERER'S STONE (2001)
Composer John Williams composed the iconic "Hedwig's Theme" before seeing a single frame of the movie. Warner Brothers was putting together a promotion that needed music, so they asked Williams to quickly write something. Isn't that crazy given that "Hedwig's Theme" is now one of the most recognizable pieces of movie score? I'd say it's right up there with Chariots of Fire and Star Wars.
Take a listen: