> The Therac-25 was the first entirely software-controlled radiotherapy device. As that quote from Jacky above points out: most such systems use hardware interlocks to prevent the beam from firing when the targets are not properly configured. The Therac-25 did not.

This makes me think - there was only one developer there, I guess, who was doing everything in assembly. This software, and the process to produce it, must have been designed in the early days of their devices, when there would be expected to be hardware interlocks to prevent any of the really bad failure modes. I bet they never did change much of the software, or their procedures for developing, testing, qualifying, and releasing it in light of the change from relying on hardware interlocks to the quality of the software being the only thing preventing something terrible from happening.