I know David has signed off, so this reply is directed to others who may come across this discussion later.
I attempted to recreate what is shown in his image using SU 2016. I don’t know the exact sequence of steps he followed. Here’s what I did:
- I created guide lines 2mm away from the red and green axes.
- I drew two edges starting at the origin and 2mm long, one along red and one along green
- I activated the arc tool, picked the intersection of the guides as center, and swung an arc from the end of one of the edges to the end of the other edge.
Below is what I got. You can see that the ends of both edges got “sucked” over to the next vertex of the arc. That is the sort of thing that happens at very small scale in SketchUp. You can’t tell from the image, but this also caused the arc to be decomposed into a series of edges.
Then I modified the previous result by deleting the two (now errant) edges. I noticed while I did so that the small edge cleanup caused the former arc to have a longer segment at each end. That is, the two short edges were replaced with a single longer one. Subsequently I could redraw the two on-axis edges to get the following, which looks just like David’s image. You can plainly see the elongated start and end edges, as you can in David’s image:
So, here is what happened: the original edges were within SketchUp’s nearby vertex tolerance of an arc vertex as the arc was generated. So SketchUp concluded they were meant to intersect and moved the edge’s end to the vertex on the arc, replacing the single edge with two in a slight kink. Then when I deleted the (errant) edges, SketchUp noticed that the prior end vertex was within tolerance of a straight line between the two adjacent former arc vertices, so it decided they were meant to be a single edge and merged them, producing the longer starting edges. I could then redraw the original on-axis edges because the vertex of the elongated former arc edge is farther away from the axis than SketchUp’s tolerance.
This kind of thing may not happen to you every time, but it is lurking out there, is very hard to detect unless you look extremely closely, and can cause downstream glitches in your model. That is why they added the trap in SU 2017.