Sounds like a missing road segment. I experience that sometimes even in Garmin City Navigator.
In these cases I convert the route to a track and go in and edit the track to fix the problem and then navigate using the track. Like the guy said you have to be smarter than the machine.
I looked into OSM a year or so ago. I checked dirt roads in some remote areas I know pretty well and found a few errors. It was enough to make me not want to rely on these maps in remote areas that I don't know.
And my reaction was "You get what you pay for".