Let's say we have city with lots and lots of roads on it, and the non-road area is filled with grass. and we have a car that should move randomly in that city, and that's easy using the pathfinding behavior, but there is a little problem, our car can find a path to the grass. Still have a solution which is adding the grass as an obstacle, but that makes a new problem, cause when it finds a path to an obstacle, it will search for the nearest non-obstacle area, and it can be at the edge of the road which will block the path of other cars, and cars will start to hit their each other... Huge mess isn't it ?
Please help.