For some reason I can’t cap or cut the vertical extrusion, and I have no idea why (picture shows how the surface buckles). To me it looks like I extruded from a clean curve etc, but the resulting surface wants to go bananas when trying to cut or cap it.
The red curve is unusual. It is degree 3 but has discontinuous curvature, presumable because it has multiknots. Explode it and two degree 3 curves result; one curve with a long segment and the two ends, and the other a long segment only. The long segment and two ends has discontinuous curvature with the multiknots. RemoveMultiknots drastically changes the shape of the curve.
The red curve was drawn from top view as a Control Point Curve. However, one curve end turned out to not being located on the plane (it was sligtly above) so I projected the curve down to the plane (and probably ran FitCrv on it as well). Perhaps that could explain why it causes surfaces going bananas.
I opened the model and it looks like a typical rendering mesh failure that Rhino has since many years. Recently I mentioned this to Pascal. This happens on surfaces with Degree 1. There are two simple solutions, depending what you are looking for:
If you select the model, open the Properties window and apply a custom mesh with “Minimum aspect ratio” like 6 or so, the problem with wrong rendering disappears.
To completely get rid of this problem without using a custom render mesh, you simply have to use the “Merge surfaces” command with the “Smooth=No” and “Roundness=1” options. Then both surfaces will be transformed into one that you can cap and it will not have any issues with the rendering mesh.