What to do if I have something missed on an x-ray, mri, CT scan etc?

Inform physician. Radiologists trained in imaging from residencies and fellowships usually read imaging studies. They are usually peer reviewed to avoid making errors. However they are human and can make errors.Error rate is usually universally about 3-4%. In order to improve performance of medicine providers need to be informed. In imaging as in medicine there are gray areas and everything is not black and white.