Particularly when I chose the fast route this always leads to problems. Yesterday I was again in front of a forbidden farmer track. On OSM it was implemented as an unpaved track with ref L2113. (I changed it to paved and "prohibited") You can try "Vennbahn (Tourist Information)" in Germany as destination. This POI is not accesible via regular roads. It would be nice to end the route at the place where the asphalt ends and give a hint that the destination is not accesible.