The Therac-25 Incident (2021)

> software quality doesn't appear because you have good developers. It's the end result of a process, and that process informs both your software development practices, but also your testing. Your management. Even your sales and servicing.

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...

Honestly I wish instead of the Therac-25, we were discussing a system which made use of unit testing and defensive coding, yet still failed. That would be more educational. It's too easy to look at the Therac-25 and think "I would never write a mess like that".

One instance that crosses my mind often is the airbus a320 incident at Hamburg in 2008. Everything was done right there, but the requirements were wrong.

Despite all the procedures and tests, the software still managed to endanger the lives of the passengers.