Researchers from the University of California at San Franscisco and the university of North Carolina at Chapel Hill discovered alarming results when they interviewed 1697 people who had been seen as outpatients at seven health care centres in North Carolina during 2008.

227 (13.4%)  people reported a wrong diagnosis, 212 (12.5%) people received the wrong treatment, 265 (15.6) reported a mistake by the physician and 8 percent perceived a “lot” or “severe harm” from the perceived mistake.

In many of these cases the patients don't complain or sue the physician they simply leave their doctors.

http://archinte.ama-assn.org/cgi/content/abstract/170/16/1480, http://www.fiercepracticemanagement.com/story/patients-leave-doctors-over-perceived-physician-errors/2010-09-14