A good example of a shading error where the vertex normals are ignored is where a round fillet joins on to a flat section (a very common occurence in mechanical objects). The Phong shader will treat the flat section as an extension of the fillet and you will get some odd shading occurring across the flat part. Without vertex normals, the solution is to add a thin ring of co-planar polygons round the flat section to "protect" it. This is normal practice when modelling directly in polys but is not done when exporting from Nurbs, like MOI.
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