At the very end of the Munchkinland scene, when Dorothy is dancing off out of Munchkinland (actually, into the backdrop), the Munchkins escort her to the edge of town. Notice that the border of Munchkinland is marked off by a row of large white flowers. Watch the Munchkin on the far right (in the view from behind the Munchkins)--he jumps up and down, and as he does so, you can see the flower border right through him. Apparently the flowers were filmed separately and matted into the main shot.