Hi Martin, from what you're showing there that's not quite a "self intersection" like when a surface is actually crossing back over its own self.

What you're showing there is more of a "edge overlapping" intersection. Those can work ok if you've got pretty precise geometry, like for instance doing a union on the attached 3DM file seems to work ok.

But when you have something that has edges barely grazing each other and it's constructed from some kind of fitting process like sweeps, what can happen is that in actuality the edge is sort of barely wiggling back and forth above and below the surface and that kind of thing becomes quite difficult for the intersector to get a clean intersection from.

- Michael