In Exact Surfaces>Edit>Edit Contours
You can click on the Check Problems box to find out if there are any errors with the contour layout.
A Region Error usually occurs if there are contour lines that connect to a boundary along an edge. This is usually a very small area that is too small or odd shaped to put a surface on.
Region Errors can also occur if there are intersecting contour lines (creating another region that's not technically defined by contours). If these areas are present, zoom into the suspect area and drag the contours around to remove the region.