Hi David, yeah only one of those cutting curves seems to be ok - the others appear to have a small area of self-intersection in the curve.

Notice what happens here if I turn on control points and pull them around:



See that one area of the curve where it kind of pulls out differently? That's an area where there is a tiny additional segment in the curve.

If you zoom in here and find these points:



Moving that middle point down shows that it's not just a micro segment, but that it actually kind of zig-zags backwards for a tiny amount before continuing forwards - that's the kind of self-intersection that will confuse things that involve intersections:



It's hard to know how that happened without seeing some of the earlier steps in how it was created - stuff like that can happen when 2 curves are barely grazing each other and are close to being tangent to one another where they are closest to each other but not quite - that kind of "almost tangent but not quite" situation can cause little bits left over when you try to trim stuff like that with each other.

- Michael