Why Did Therac-25 Fail?
The Software Bug That Killed Four Patients — and Taught Medicine About Checklists
📜 What Happened
The Therac-25 was a radiation therapy machine that seemed safer than its predecessors because its safety was implemented in software instead of hardware interlocks. But when the software hit a rare race condition, the machine delivered massive overdoses of radiation — silently, and repeatedly. Over two years, six patients were over-irradiated; four died. The horrifying part: the machine's logs showed 'no malfunction' even as patients screamed — because the software recorded normal operation while the hardware delivered death. The investigation found a system designed by brilliance and tested by assumption, with no independent verification and no checklists.
☠️ The Fatal Mistake
Trusting complex software without layers of redundant human and hardware checks — genius engineers assumed what a simple checklist would have caught.
🧠 The Lesson (Free for You)
Complexity exceeds any single expert's memory — even brilliant ones. The aviation industry knew this for decades; medicine learned it the hard way. Critical systems need checklists, verification and the humility to assume you missed something.
📕 The Antidote Book
Surgeon Atul Gawande asked a strange question: why do experts still fail at complex tasks, from surgery to skyscrapers to airplane crashes? His answer: modern complexity exceeds any single human's memory — the failures…
📖 OPEN THE FULL INTERACTIVE BREAKDOWN →Searchable, filterable, free to read — they paid billions; your lesson is free.