How to detect: The 3D Bent Tube is misplaced in the 3D View and if you look at the projections of the Bent Tube on photos where it was not marked, it does not project onto the current real-world tube.
How to solve: The most probable cause is that the tube marks are misreferenced. Check to ensure the Bent Tube marks that are referenced are all of the same real-world tube. In addition, the all the marks may be of the same tube but key parts are not marked (for example with a “U” shaped tube, one ‘leg’ is marking in one photo but not the other, but the other ‘leg’ is marked in this other photo). This can confuse the algorithm because the matching is ambiguous. Fix by covering more of the tube with appropriate marked edges. You should usually need no more than 4 to 5 marks for a tube. Start with 2 marks and add a 3rd if you need to cover more sections.