Tim.
I was plotting a route:
Caernarfon - Hawarden - Shobden - Aberporth - Caernarfon.
I plotted the route ok, but as the last leg crosses D201, I dragged it inland to one of the wind farms for a nav point. As I zoomed in using the mouse wheel to make sure it lined up with what was on my map, then I got the error.