55 years ago, Dr. F. Mason Sones, a Cleveland Clinic cardiologist, thought he had made a fatal mistake when he accidentally injected dye directly into his patient’s heart. But what seemed like a grave medical error in 1958 proved to be a monumental breakthrough. His ‘error’ helped launch the modern era of  heart disease diagnosis, and allowed Dr. Sones to become a pioneer in what is now called coronary angiography, which allows doctors to see inside the heart and blood vessels.