My father's case is an interesting one. For the first few routine colonoscopies, in which benign polyps were found, he had an incompetent gastroenterologist perform the colonoscopy only part of the way. He did not view the ascending colon, where it joins with the small intestine. Several years later, the first time my father had a colonoscopy by a different doctor, a longstanding tumor was found, right in the section that the previous doctor avoided viewing.
I don't know why this would happen, unless that first doctor was really that incompetent/negligent; or, if some doctors avoid making that last bend, with the scope, in the event the rest of the colon is clear. Maybe this is because of added risk? It just seems stupid and unfortunate. He needlessly ended up with colon cancer, had the first doctor found the original offending polyp and removed it. At some point, it was probably benign.