Continuity mistake: Nicholas Hammond (Friedrich) was not a natural blond. The first scene to be shot involving him was the "My Favourite Things" scene and you can see that his hair is pale auburn. After this scene was shot, the directors thought that he would look better if his hair were lighter, so they applied an early form of bleaching agent to his hair. In scenes which were shot after this (notably the children's first entrance to meet Maria which occurs before the "Favourite Things" scene) his hair is very, VERY blond, almost white. In fact, if you scrutinise his hair closely throughout the film, it is a different colour in practically every scene, depending on when the scene was shot and how much of the bleaching agent had been applied.
Continuity mistake: During the boat scene, the baroness leaves and walks behind Maria. From the immediate opposite shot the baroness has vanished.
Answer: In the film, the Mother Abess explains to Maria that the Captain Von Trapp's wife died "several years" ago. This is commonly misheard by viewers as "seven years." In reality, the actual mother of the Von Trapp children was Agathe Whitehead, who died of scarlet fever in 1922, just four years before Maria came to the Von Trapp home, initially as a tutor (not a governess) in 1926.
Michael Albert