Here's another problem if you try to blend with G2 continuity, which you need for cars:



This is a problem related to the other thread discussing patches with continuity. In order to get curvature continuity at the edge, the actual surface created is wobbly. In order to avoid this you need the edges to be exactly aligned, then cut back by some distance on each side. If one edge is on or above the virtual intersection of the surfaces, you will get a wobble.