Surface what can unfold easily in Rhino with UnRollSrf command must have a good characteric to unfold. how do I know which surface is good and what is bad. I use Curvature Analysis to determine. A good surface what can unfold with no error is a surface with zero Gaussian curvature, A zero value means the surface is flat in at least one direction. (Planes, cylinders, and cones have zero Gaussian curvature). (See picture below) surface with zero Gaussian curvature is show in green color. If your surface turn into green when analyze with Curvature Analysis that mean you can unfold it very easily and no error (see picture below). This is my technic when deal with UnRollSrf command in Rhino and hope this technic will help some modeler who want to design you own model.