ExtrudeCrv documentation file (https://docs.mcneel.com/rhino/6/help/en-us/commands/extrudecrv.htm) says: "If the input is a non-planar polycurve or a planar polycurve where the extrusion direction is not normal to the curve plane, the result will be a polysurface rather than an extrusion object."
This is not true. The attached curve is planar, but ExtrudeCrv command turns it into a polysurface. (It is called open extrusion, but if you trim it, it becomes a polysurface.)
ExtrudeCrv bug.3dm (3.2 MB)