The Yakima Software Bug (cont)
“The software problem ... is fairly well established. ...Every pass through the Set Up Test routine ...increments... a shared variable called Class3. If Class3 is nonzero, there is an inconsistency and treatment should not proceed. A zero value for Class3 indicates that the relevant parameters are consistent with treatment and the software does not inhibit the beam.”
Problem: Class3 was a 1-byte variable. Adding 1 to 255 produced 0.
Being treated with the Therac-25 radiation treatment machine was a bit like playing Russian roulette.