I believe the definitive analysis of the Therac-25 incident was written by Nancy Leveson, first in IEEE Computer,[1] and later as an appendix of her book.[2] The appendix is freely available as a PDF on the web [3][4] and probably other places. Many people here are asking questions about what happened and how it came about. The answers to many of these questions can be found there. I strongly recommend that anyone who is serious about safety and wants to learn more about this incident read Leveson’s analysis.
[1]: N. G. Leveson and C. S. Turner, "An investigation of the Therac-25 accidents," in Computer, vol. 26, no. 7, pp. 18-41, July 1993.
[2]: Nancy Leveson. Safeware: System Safety and Computers. Addison-Wesley, 1995.
[3]: http://sunnyday.mit.edu/papers/therac.pdf
[4]: https://web.mit.edu/6.033/2014/wwwdocs/papers/therac.pdf