Hi, Martin.
What we have done before in our system (note this will be different depending on your product or category), for rejector system that uses compressed air to actuate, for every time that there is a compressed air, each product is allowed to go thru after metal detector conveyor. Once there is loss of compressed air (or even atleast the pressure is less than what is required), all the product that pass thru goes to rejector bin. This is also coupled with alarm or in some set-up,include machine stoppage. You may say this may be "overkill", but you trust what you validated and even so the above, the quality losses will be mnimal.
In some case where where air is not used but using some actuator, the fail-safe is to have sensor after MD and at the rejector bin to confirm that the product is properly rejected. If one of the sensor detects unusual event, the machine is triggered to stop.
All of the prerequisites are part of the start-up check.
Again, this will be dependent on the set-up and what works for some may not work for all. It is also worthwhile to consider to check if what you have in correction will really happen as documented. Normally, it is only the control measures (for HACCP plan) that is being validated.