Hi ed, that's a display bug, it can happen if you have just a certain kind of S shape that as its start and end tangent going in the same direction. I'd like to fix that up in the future but it's a delicate area to improve without degrading performance and it's pretty rare.

See here for more detailed information:
http://moi3d.com/forum/index.php?webtag=MOI&msg=4101.2

- Michael