**not**a mistake. I hadn't considered this before. If you look at the picture here, from the same geometry as above, you can hopefully see that the two highlighted points are both incident to two quads which happen to share yet a third point. Geometrically, this should only happen if the two quads are perfectly coplanar. In practice, however, there's nothing to force a quad to be a planar polygon. A nonplanar quad is in reality the merging of two triangles defined by one of the diagonals of the quad, without specifying which. It is therefore up to the Boolean sop to resolve the ambiguity as a preprocessing step and triangulate all input polygons pretty much the same way the Divide sop would do. In the example in the figure, the diagonal in questions connects the highlighted points and this creates an edge incident to three triangles, namely the one shared by both quads, and the two remaining ones, one from each quad. This would be a non-manifold edge created in preprocessing of the geometry!