The software was working just fine for years before on earlier versions with the interlocks. They never checked to see how often or why the interlocks fired before removing them. Turns out those interlocks fired often because of the same bugs.
They had two fuses, so they had a 2:1 safety margin! Just like the NASA managers who decided that 30% erosion in an O-ring designed for no erosion meant a 3:1 safety margin.
