Some surface geometry can contain degenerate patches, singularities, or self-intersections. These types of surfaces could potentially be produced with the Reshape tool, Loft Surface tool, or Create Surface from Curves command, and can produce undesirable results in the finished model. Surface operations, such as trim and stitch surfaces, split, and other operations, like creating contours and solid operations, may not be able to manipulate these types of surfaces.
Decomposing such surfaces may help by separating the surfaces into NURBS surfaces without discontinuities. See Decomposing Objects and Surfaces.
~~~~~~~~~~~~~~~~~~~~~~~~~