Steps to reproduce:
- Create a curve and select it.
- Filter for “control points” only.
- Select a few control points of the curve (note how the entire curve is still highlighted too).
- Project onto a surface without changing the selection (note how the entire curve is projected).
What happens now if you leave the filter on is that error message.
However, if you are careful to turn the filter off before step 5, you still don’t get anything isolated, yet it appears as if both the projection and the original curves are selected (and the gumball is placed in between both the original curve and the projected result).