A coronary angiogram is a test for blockages in the heart's blood vessels. It involves injected a considerable amount of dye into those vessels. The first such test was an accident; although the patient wasn't in any physical danger, it's a surprise he didn't die of fear.
If you've had serious heart trouble, it's likely you've had an angiogram. It's an unpleasant but safe test that involves inserting a catheter into the blood vessels into your heart and sending in some special dye. The dye can be seen in x-rays, and as it moves through the heart, it allows doctors to see possible deformations and blockages. Up until 1958 it had only been used to view the chambers of the heart. Then a doctor's assistant had a leeeeetle accident.
Doctors thought that injecting the dye into the arteries around the heart would cause the patient to come down with a bad case of death. The heart would fibrillate, and eventually shut down. So while doctors did use a catheter to inject dye into the aorta, they left the blood vessels alone.
When Doctor Mason Sones' assistant was injecting the dye, the tubing slipped and about 20 milliliters went directly into the coronary artery. The patient's heart stopped beating – possibly because of the dye, but more likely because Sones screamed out, "We've killed him!" (It's unknown if Sones ever played poker, but if he did, he probably died in poverty. Keep that face steady, doctor!)
After the patient, who was only 26 years old, got over his shock, his heart kept beating and Sones realized that this accident could be turned into a useful test. He slowly worked out a procedure that has become an angiogram. He probably also had to attend a couple of classes on maintaining a proper bedside manner.