Continuing the discussion from What steps move circle to be tangent to another?:
Here’s a worked example starting from very simple geometry which shows how an inaccurate trim prevents two resulting arcs from joining. In comparison, split and intersect both hit the spot correctly.
This problem, which occurs in Rhino 5, is still present in Rhino 8 today. It’s a real PITA.
Trim Curve Inaccurate - example.3dm (91.3 KB)