The Therac-25 Incident (2021)

If you only take one thing away from this article, it should be this one! The Therac-25 incident is a horrifying and important part of software history, it's really easy to think type-systems, unit-testing and defensive-coding can solve all software problems. They definitely can help a lot, but the real failure in the story of the Therac-25 from my understanding, is that it took far too long for incidents to be reported, investigated and fixed.

There was a great Cautionary Tales podcast about the device recently[0], one thing mentioned was that, even aside from the catasrophic accidents, Therac-25 machines were routinely seen by users to show unexplained errors, but these issues never made it to the desk of someone who might fix it.

[0] https://timharford.com/2025/07/cautionary-tales-captain-kirk...

I have years of experience at Boeing designing aircraft parts. The guiding principle is that no single failure should cause an accident.

The way to accomplish this is not "write quality software", nor is it "test the software thoroughly". The idea is "assume the software does the worst possible thing. Then make sure that there's an independent system that will prevent that worst case."

For the Therac-25, that means a detector of the amount of radiation being generated, which will cut it off if it exceeds a safe value. I'd also add that the radiation generator be physically incapable of generating excessive radiation.

I agree with the previous poster that the feedback from the field is lacking a lot. A lot of doctors don’t report problems back because they are used to bad interfaces. And then the feedback gets filtered through several layers of sales reps and product management. So a lot of info gets lost and fixes that could be simple won’t get done.

In general when you work in medical you are so overwhelmed by documentation and regulation that there isn’t much time left to do proper engineering. The FDA mostly looks at documentation done right and less at product done right.