I have run into this problem many times in the many years I have used Maya. The mesh pinches in an non-intuitive way when I press 3 (preview smooth mesh). It should just stay smooth but instead it pinches on the edge. The issue is not duplictaed edges, not extra vertices, not hard edges, not creased edges, not related to UVs and as far as I can tell not a normals problem. I usually just delete the problem faces then append polygon to fix this problem but I would love to know if there is an easier solution? Is there a modeling workflow to avoid this problem?