Flying drones
FBW systems are very complex. Such a sophisticated piece of software also switches modes or operation states according to several constraints in real-time.
However, it is impossible for any system to deliver the expected output or best solution in an absolute way when the inputs are given by natural forces (non-finite domain). You can test the constraints by generating
variables to feed the different modules, but the sheer complexity and the amount of possible values implies the use of heuristics to deal with certain combination of inputs.
Upper and lower limits may produce an undesired behavior given unexpected situations like: a stall warning which is triggered when the pilot is actually solving the problem by feeding nose-down inputs.
At the other hand, those limits may save the aircraft in other scenario.
So, there more complex the system is, there more prone it will be to deliver unexpected outputs given a real-life situation.
At the end, AF447 was an almost fully operational aircraft that ended at the bottom of the ocean.
Are the highly trained and experienced pilots to be blamed?
Why? Because they got confused?
Ask yourself a single and simple question: Confused by whom?