Hi Rogurt, also see this other post here:

http://moi3d.com/forum/index.php?webtag=MOI&msg=1127.4

For a description and illustration of what a degenerate corner on a surface looks like.

When a surface has its own control point grid organized in this way it is easy for the surface to become self intersecting as shown there, and that basically creates a micro tiny fold in the surface and that means the surface normal (which is a key thing used in offsets) can flip around wildly in that little spot.

So rather than making a surface that has that kind of degenerate corner in it, it will be better for offset to have a larger surface that has its control points in a more regular grid pattern instead of having corners of the grid bunched or stretched, and then have a trim curve that cuts away the excess area of the surface.

You can have all kinds of complex trim curves on a surface without it affecting the surface normal, but if the surface's own control points are bunched up or folded back on themselves it is a no-go for offsetting that.

- Michael