The third mistake was that, because the workers were allowed to bypass the time-consuming mixing process in the tower, with its automatic controls for preventing such things happening, they were free to dump their material into a vessel that was not designed to prevent the accumulation of a critical mass sufficient to support a chain reaction.
Second, every time I put the lid on the mixing bowl, which is what presses the safety button that allows the motor to spin, I felt like I had to force it a little.