Update comment

src/kernel/shapes/sketch.rs

@@ -68,9 +68,23 @@ impl Shape for fj::Sketch {
         let vertices = self
             .to_points()
             .into_iter()
-            // These calls to `create_at` are valid, since the points of this
-            // sketch come directly from the user and define new vertices of the
-            // shape.
+            // These calls to `Vertex::create_at` don't follow the rules that
+            // the documentation of `create_at` lays out. This method can be
+            // called by an outside caller, and is additionally called by other
+            // methods in this trait implementation, either directly or
+            // indirectly.
+            //
+            // This means the same vertices are re-created multiple times, which
+            // is forbidden. I don't think this is causing any actual problems,
+            // since these `Vertex` instances are created from points that come
+            // directly from the user, and aren't being computed here.
+            //
+            // But still, this rule exists for a reason: to prevent subtle bugs
+            // from creeping in. We should follow it, here and everywhere.
+            //
+            // Please refer to this issue for more context on the problem, as
+            // well as a proposed solution:
+            // https://github.com/hannobraun/Fornjot/issues/176
             .map(|[x, y]| Vertex::create_at(Point::from([x, y, 0.])))
             .collect();
         Vertices(vertices)