Originally Posted by
Abrahn
The scenario you quoted wasn't an extreme edge case. The system was specified to be able to deal with 193 controllers but was only tested with 130. And broke at 153.
To use your analogy that's buying a 4 seater car and only bothering to check that there are 3 seats in it.
There was a little bit more to it than that. The other issue at play was that the controller had made a mode error in selecting a soft key that put them in "Watching Mode" (a rare and obsolete mode) and only
then did the comparison 153 < 151 (in a different code path) fail. It was the combination of errors both in software and by the operator that, on their own were inconsequential, but when combined became significant.
The final report paras ES8. and ES9. give an introduction to this. The report then goes on to look at why this mode was still present, how this (understandable) mode error could have been detected (it was being selected accidentally almost every other day) or prevented and the trade-offs in testing and so on.
Much of software testing is about using your imagination; "what can go wrong?" so the 2014 failure could be regarded as failure in imagination.