An article from Auntie Spam entitled Rental Car GPS Leads to Nightmare Vacation, details the story of a family that rented a vehicle in New Zealand and ended up on a harrowing bit of single track road.
Sometimes, there's no substitute for a good map. And, although a GPS is a fine device, they can have optimistic views of what the road network actually looks like. It's unclear from the story if the problem is pre-dated road upgrade information or settings on the GPS that indicated they would be happy with single track.
Either way, it's a good reminder to keep a map close and to pay attention to the road signs.