Extrusion does not work well on closed splines
Consider the attached geometry containing a closed B-spline, that starts and ends at the bottom point. In 2D, the geometry is correct (first image). Now when the B-spline gets extruded, the endpoint gets extruded as well (second image). This seems to create a topological problem with the extruded entities, as it messes up the surface mesh and generates an error (third image).
The error also occurs with closed interpolating splines, unless the mesh is extruded as well (but mesh extrusion does not fix things with B-splines).
@mrasquin and I have hit this bug several times. Splitting the spline is a possible workaround, but is not does not provide the smoothness required in high-order meshes to be used in DNS studies.