One of Atul Gawande's books (Complications) mentions that amongst deaths where an autopsy is done, the cause of death was misdiagnosed by the doctor about 40% of the time. Quote from book [Page 197, Chapter name : "Final Cut"] ---

"How often do autopsies turn up a major misdiagnosis in the cause of death? I would have guessed this happened rarely, in 1 or 2 percent of cases at most. According to three studies done in 1998 and 1999, however, the figure is about 40 percent."