Hi Metin, it's hard to see since it's in such a tiny area but basically the end of your surface there folds back over itself, making a tiny self intersecting area. That kind of sudden fold in a tiny area will make Blend go crazy since it tries to maintain things perpendicular to the surface edge, and the surface form changes very suddenly in shape in a tiny area there.
If you zoom in very closely to the end area you'll see some strangely shaded stuff like this:
This kind of weird surface anomaly can happen with a scaling rail if your scaling rail ends a tiny distance before the end of the sweep, it can be good to actually extend the scaling rail out a ways past the rail to make sure it totally covers the full span of the rail and doesn't leave any uncovered area where there will be a sudden shift in shape.
Check out here for some previous discussion on scaling rails:
