Look at it when Dorothy is about to discover the Scarecrow, well he is behind a fence that seems pretty OK to me. Then, what has happened? the fence seems to be broken??
Well, the broken fence is a product of another deleted scene. Originally, the scarecrow's song and dance sequence was much longer but they cut it out because they said the movie was already too long. And in his original dance, he fell into the fence and broke it.