Maya - How to remove the "hidden twist" from curves?

Hey all,

got a recurring problem when using curves to build geometry, whether using motion paths, extrudes or Mop's renderable curve script. Frequently a seemingly smooth curve will have a twist in it that only becomes apparent after using it to create geometry -



I've tried all sorts of stuff to prevent this, including rebuild curve etc. Does anyone know a foolproof way to stop it?

edit : this only occurs with curves that have an "S" bend in them.