I've extruded a sphere which I later deform with a shape-keyed lattice. All's well, except that when deformed, I see a discontinuity in the shading. Inspecting the face normals, I see the following:
You can see that there's a loop which has two odd characteristics:
It has both edge and face markers;
It has no normals (and they're not just pointing the other way, they really appear to be absent).
Selecting everything and recalculating the normals had no effect.
If I select an edge segment along that loop and delete the edge, one of the adjacent faces gets deleted too.
I think this is an edge from which I extruded to lengthen the tube.
How can I repair this loop?