The photo "PCB v0.1 good and bad together.jpg" shows a good example of what can go wrong port II.

I had chosen a smaller size for auto leveling than the PCB had been. I had increase X-Min, because  near the top (Y) the milling bit would be outside the PCBmaterial.
And I had ignored review the result of the auto leveling the 3D view.

In result, bCNC had calculated the surface  around the edge of X-Min is higher ... so the mill did not touched the 10mm strip on the left (X-Min).