Hi Tony, it is a bug, it happens specifically if you have a helix with 7 twists in it.

It happens that the sweep tests at 7 spots along the surface to see if the generated surface is coincident with the sweep profile at that spot, to see whether it needs further refinement or not. It so happens that the flat surface you show there actually meets that testing.

Please see this thread: http://moi3d.com/forum/index.php?webtag=MOI&msg=1263.1
for a workaround.

- Michael