Go to the scene where the Wicked Witch is just leaving Munchkinland. As she starts to disappear in the smoke, she gets burnt. You may think that one of the Munchkin's scream was her scream. I thought that myself. She didn't know she was burnt until after the scene. The crew set off the flames too early!
It's common knowledge that the woman who played the Wicked Witch, Margaret Hamilton was severely burned during the movie with her entrances and exits. Try something a little less known.
It's true that the wicked witch actress, Margaret Hamilton, was severely burned in an accident during the filming of that scene. However, the scene which made it to the movie was not the one in which she got hurt, but the one filmed beforehand. (Or so said Margaret Hamilton in her memoirs.) We're not witnessing a live witch-burning here! :)
After the witch recovered from her first burns, she was burned a second time. The second time was during the sky writing scene. The tail of her dress caught in the tail pipe of the smoke thing she was sitting on, and it caught on fire. One of my teachers is a Wizard or Oz buff and she told us all kinds of stories about the making.
Actually, on the DVD they have interview-type things with some of the cast. On the one with Margaret Hamilton she says herself that the scene that made it to the movie IS the one where she got burned going down. Jack Haley, the guy who played the tin-man, pulled her away from the fire.
Margaret Hamilton herself, in many interviews, states that on the morning of the filming, they filmed the scene and it went fine, and that's the one in the movie. In the afternoon, they filmed again, and everyone was tired, etc and that's when the burning accident happened.