Moving ground_objects can be trapped in cities, where they can't move properly anymore. If that happens, the object will attempt to move in a random direction. Once it reaches the end of the tile, since the tile in the direction it moved to is not appropriate/free, it will teleport back in it's initial position and try again.
I wonder if it would be much work to have it turn around instead of teleporting, so it would enter the tile where it would have left it. That way, even an object trapped on a single tile could move in 3 different directions from each position, allowing for smooth and varied paths even in confined space.