The Angle error can occur after clicking on the Check Problems button in Exact Surfaces>Contours>Edit Contours>Edit Extensions.
This error occurs when there is an angle that is not ideal for building a surface on because the Grid lines could become compressed resulting in a wrinkle in the surface. Below is an example of the error occurring. It appears the small angle, indicated by the arrow, needs to be adjusted to fix the error.
Adding a point by left-clicking on the line and then dragging it to make a bigger angle will create a better surface contour layout. Click Check Problems again to see if the error has been fixed.