A clear example of a machine which is a program with physical appendages is the Therac-25, a radiation therapy machine built by Atomic Energy of Canada Ltd. and available from late 1982. Between 1985 and 1987, six patients were massively overdosed in Ontario, Georgia, Washington, and Texas, resulting in deaths and serious injuries. The program was written in PDP 11 assembly language by one person over several years. It was a real-time multitasking system, written specifically for the machine. It was evolved from software controlling previous machines, but those machines relied on it less for safety. The manufacturer decided not to duplicate hardware safety features existing on earlier machines.

The causes of the overdoses have been analyzed by computer scientists Leveson and Turner (IEEE Computer, July, 1993, pp. 18..41), and can be ascribed to programmer ignorance of computer science and engineer ignorance of software. Computer science has known about the inherent dangers of concurrent programming since before 1961, when Edsger Dijkstra, one of the pillars of the discipline, obtained the basic solution. The engineers of the Therac-25 considered that since "programming errors have been reduced by extensive testing .. any residual software errors are not included in the [safety] analysis" conducted a few months after the machines became available.