What to do when GH doesn't find a correct section...?

In the attached, I'm just making a slicing component as part of a larger thing, using plane/Brep intersection.  Problem is I'm getting unpredictable results on things like boxes parallel to the slice direction - one or more of the concurrent face sections is not found correctly...  Insetad of a closed polyline which represents the face, I get some edges duplicated and joined together incorrectly and a single line segment left over...

I know Rhino's contour tool has problems, especially in V4, but it doesn't produce a bad result in this case, whereas GH does.... 

OK, never mind, it works better in V5 - guess I'll just have to work in V5 on this one.  As I already wrote all this before testing in V5, problem included for reference anyway...  --Mitch