There’s an old joke: what would happen if programmers became doctors?
“Doctor! My leg hurts!”
“Strange, I have the same kind of leg and mine doesn’t hurt.”
Few people know that this actually happened. An icy horror grips my heart when I reread the records of those tragic events of 1985–1986 in the United States.
The Americans developed a radiation therapy machine: the Therac-25. As a result of various errors in the machine’s software, six people received an excessive dose of radiation; at least two died from the overdose.
On March 21, the patient Ray Cox was prescribed electron therapy. The nurse accidentally entered “x” (X-ray) — X-ray therapy was used more often, and she was used to it. Then, returning with the “Up” key, she replaced “x” with “e” and pressed “CR” several times. A few seconds after the radiation started, the machine stopped with the message “Malfunction 54.” Similar shutdowns had happened before, but their only consequence had been a minor inconvenience. So she pressed “P.” On this particular day the video link was off, and the intercom was broken, so there was no communication with the patient.
At that moment the patient felt a burn. Since this was not his first procedure (his ninth), he realized something was wrong. Just as he began to get up from the table, the beam turned on and burned his arm again. Ray got up, staggered to the entrance to the room, and began knocking on the door. Hearing the knocking, the operator immediately opened the door to the shocked and injured patient.
The total radiation dose received by the patient is estimated at 16,500 to 25,000 rad, and it was delivered in less than a second, over an area of about 1 cm².
Later, Ray Cox began suffering periodic bouts of nausea, and his left arm became paralyzed. He was hospitalized because of radiation exposure. Still more severe complications followed (including paralysis of the legs), and the patient died in September of that same year.
Source: Therac-25
A detailed report including all overdose cases, investigations, and causes of the errors: http://sunnyday.mit.edu/papers/therac.pdf