Therac-25

A radiation therapy machine involved in at least six massive overdoses between 1985 and 1987; three of the patients later died. It dropped the hardware interlocks of earlier models and relied on software reused from them, where those interlocks had masked the same defects. One fault was a race condition triggered when experienced operators edited settings quickly. Another came from a flag set by incrementing a one-byte counter, which occasionally overflowed to zero and skipped a safety check. The cryptic 'Malfunction 54' message gave operators nothing to act on, and early reports were dismissed. Nancy Leveson and Clark Turner's investigation is still the clearest account of why safety belongs to the whole system.