Lee's terrain system is fantastic, I have learnt so much about unity by following along through his r&d sessions.

Here is a fix i made for when you fall through the floor, avoiding the use of terrain.GetHeight(x, z) which uses the heightmap info to determin the terrain height. This will ignore any sculpting you have made.

Add this to TerrainDemo.cs Update() method.

Code:
            var pos = TP_Controller.Instance.transform.position;
            var ray = new Ray(new Vector3(pos.x, 5000, pos.z), Vector3.down);
            RaycastHit hitInfo;
            
            if (Physics.Raycast(ray, out hitInfo))
            {
                if (hitInfo.collider.tag == "Terrain")
                {
                    if (pos.y <= hitInfo.point.y - 1)
                        TP_Controller.Instance.transform.position = new Vector3(pos.x, hitInfo.point.y + 5, pos.z);
                }
            }