<P> Previous models had hardware interlocks to prevent such faults, but the Therac - 25 had removed them, depending instead on software checks for safety . </P> <P> The high - current electron beam struck the patients with approximately 100 times the intended dose of radiation, and over a narrower area, delivering a potentially lethal dose of beta radiation . The feeling was described by patient Ray Cox as "an intense electric shock", causing him to scream and run out of the treatment room . Several days later, radiation burns appeared, and the patients showed the symptoms of radiation poisoning; in three cases, the injured patients later died as a result of the overdose . </P> <P> A commission concluded that the primary reason should be attributed to the bad software design and development practices, and not explicitly to several coding errors that were found . In particular, the software was designed so that it was realistically impossible to test it in a clean automated way . </P> <P> Researchers who investigated the accidents found several contributing causes . These included the following institutional causes: </P>

What is the therac 25 what kind of bug did it have