Any image that has a consistent stripe opposite to the direction being reduced will be found and reduced. The problem is, when this stripe involves the image trying to be preserved, it object will be carved and distorted by the reduction process. In this case, dark parts of the windmill get included as part of the sky and removed. Of course, this also means that seam carving only really works well on scenes with a large background or a consistent pattern. It does some cool things to other scenes. See below :)