Many patients died from accidental overdoses of radiation. There are numerous papaers on the Therac-25 problem. The designers of the 25 felt that hardware protection such as overdose sensors, etc. were a waste of money since all safety checks could easily be done in software. The hardware which protected against accidental overdoses on the 15 and 20 was eliminated on the 25. Consider the lowly microwave oven.
Every microwave uses hardware to protect the user against accidental exposure.