The top image shows two surfaces, sectioned using the clip cube. The one on the left is a mesh; the one on the right is a NURBS surface. In the OpenGL clip cube, the section is correct.   However, in a section viewport, they are not sectioned correctly, with an extra line joining the two ends of the line that represents the section through the surface. This is shown in the second image.   I have attached the file.   surfacesect.vwx