b$.$ A linear electron accelerator for therapeutic use was built as a dual$-$mode system that could either produce $x-$rays or electron beams. It had been in successful use for some time, but every now and then some patients received high overdoses, resulting in painful after$-$effects and several deaths. One patient on a repeat visit experienced great pain, but the remotely located operator was unaware of any problem because of lack of communication between them: The intercom was broken, and the video monitor had been unplugged. There also was no way for the patient to exit the examination chamber without help from the outside, and hence the hospital was partly at fault. On cursory examination of the machine, the manufacturer insisted that the computerized and automatic control system could not possibly have malfunctioned and that no one should spread unproven and potentially libelous information about the design. It was the painstaking, day$-$and$-$night effort of the hospital's physicist that finally traced the problem to a software error introduced by the manufacturer's efforts to make the machine more, user$-$friendly. ${?}^8$