How to detect: In the 3D View the Bent Tube does not look very straight or form a nice curve.
How to solve: The two main reasons for Bent Tubes with poor shape are low photo angles and poor marking. To correct add or replace references on photos with better angles for this Bent Tube (rotate around and big enough angle between), and zoom in and carefully match the markings to the real tube silhouette (move edge or width points, add and delete points, etc. to get both edges to match the tube edge well in all photos).