What does the "Quad" error mean in Edit Extensions?

In Exact Surfaces>Edit Contours>Edit Extensions

Click on the Check Problems box to find out if there are any errors with the extension layout.

Quad means that the angle between the two sections of the contour divided by the node is not acceptable for fitting (usually more than 180 deg). Creating spring curves at that corner usually helps. Try moving the vertex a little and click on the Check Problems button again.

Below is an example of such an error. Notice the large angle on the left side of the vertex.

After moving the vertex to fix the angle, click Check Problems again to see that the error is fixed.