Hi Ed, thanks for the bug report.

It looks like the problem you are seeing there is actually just a display artifact, the geometry itself appears to be fine.

Please check out this recent post that talks about how the display works with a triangle approximation being created to produce the shaded display:
http://moi3d.com/forum/index.php?webtag=MOI&msg=2725.4

The case you have here just did not get enough triangles produced in it to make a nice looking result. Your actual model geometry is all proper though.

It can be tough for the mesher to pick up on things that are long and only very shallowly curved like you've got here.

Are you going to export this to a polygon mesh file?

If so then you may need to adjust one additional parameter when doing so, since the curvature is very shallow the regular angle metric won't really dice up that shallow area, so you may need to enter in a value in the "Divide larger than" option in order to dice long areas up into smaller pieces. For example putting in a value of 2 for "Divide larger than" will produce a good result on your particular shape there.

In the future I would like to adjust some things to get a bit better polygons out automatically in this kind of "long but only shallowly curved" type shape situation.

But for now, you just ignore it if you see it in the viewport, and use "Divide larger than" to get rid of it at export time.

- Michael