Hi Paul, you're not doing anything wrong, it is MoI that is getting confused in that situation.

When you created the construction line on top of the original line, it basically is hiding the original line as far as object snapping is concerned.

In order to add a Trim point, Trim depends on the object snap being attached to the object being trimmed - in this case the object snap is instead targeting the construction line and MoI is not seeing it as attached to the original curve. Even though it is on the right spot, Trim is not seeing it being on the correct object.

I think I should be able to improve this for V2.

In the meantime, you may have to create additional geometry for this case (construction line directly over top of a regular line inside of Edit/Add trim points) - if you place a point object there, you can trim and use the point object as the trimming object and that should work.

- Michael