I think there are two points here.
1) If you believe that in the environment in which it will be used, your product may be subject to levels in excess of that defined in the standard, you must test to that higher level.
2) Yes, failures can occur at mid range levels. We have just recently experienced this problem with ESD. Below about 3.5 kV and above about 4.5 kV the product worked fine. But at 4 kV we experienced a failure mode. If we had only tested at 8 kV we would have missed the problem.
I also had a problem some years ago with mains voltage dips to 0V. Having a test set that enabled the period to be varied in ms increments, I discovered a problem affecting a voltage regulator. When the mains dip was between about 16 ms and 35 ms, the regulator went into a bistable mode switching on then off at each pulse. Outside these periods, it worked satisfactorily, eventually losing output when the period was extended.
The problem with investigating these types of failure is determining the size of step between measurements. Too short a step and you will be testing forever, too long and you could miss a narrow band of problem.
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