Hi Yenmonger, it looks like it is doing a boolean union instead of a difference - that usually means there is some problem with the surface normal orientation of one of the objects.

If you select them both and actually run a boolean union instead, it will actually do the difference then.

The problem one is the green object, and it looks like there may be some self-intersecting surfaces in that one which can tend to mess up intersection calculations.

Notice here at the tip where things take a kind of sudden swoop inwards:





So I think that side wall piece is actually folding back over top of itself right in that tip area - those kinds of things will tend to mess up surface intersection calculations. In this case it probably messes up the ray intersection tests that are done to determine which side of an object is the outside portion.

If you delete that one side wall face that folds over itself with a trimmed planar face instead, then it should be better, I've attached a version that is set up like that. This version should now behave normally with boolean difference.

- Michael