Prior to filming, some city officials in Salzburg didn't want to have their city decorated with Nazi flags, or have actors dressed like Nazis marching through the town square. But when the alternative suggested by the filmmakers was to use actual newsreel footage, officials changed their tune, because Austrians in the newsreels were seen enthusiastically welcoming the Nazis. (Source)

The Sound of Music was never very popular in Austria. Here's one Austrian giving us 15 reasons to hate the film.

Working with child actors can be a real problem; they have an unfortunate tendency to grow. The actor playing Friedrich grew 6 inches during the course of the filming, which caused a lot of continuity problems. Since he was supposed to be shorter than Liesl, Charmaine Carr ended up filming some scenes standing on a box. (Source)

The real Maria von Trapp was an extra in the movie. She's in the "I Have Confidence" scene. (Source)

Kym Karath, who played Gretl, swallowed so much water during the scene where everyone fell off the boat that she promptly threw up all over "Louisa." Julie Andrews was supposed to have fallen off the boat forward so she could grab Karath, who couldn't swim, but she accidentally fell backward. By the time they got to her, the poor kid was terrified. (Source)