How to detect: In a photo the mark curve (the main edge) may have strange shape, or loops. In addition if this strange curve shape happens near a width point (and the connecting line), the width point can be misplaced (width points should be perpendicular to the attached edge point.
How to solve: Insert more Bent Tube edge points into the curve to ensure the straight or bend sections are defined well. Then drag the edge points until the curve behaves.