Okay, here's a situation:
you did your QC-test*, and the result is OOS out of the blue- but you know, you really know it is a lab error. Based on your knowledge about the product and the method, you are sure about this.
So you go through your checklist, you do your interview with the analyst, you double-check everything - and you find literally nothing. You are frozen, there's no lead to follow. What can we do then?
What does MHRA OOS guidance tell us about hypothesis tests and retests?
'Hypothesis/Investigative Testing is testing performed to help confirm or discount a possible root cause i.e what might have happened that can be tested'
'If no assignable cause that could explain the results can be identified [...] retesting may be considered.'
How can we use this in the situation above?
You have a both strong and weak hypothesis: you are sure that the cause is lab error, however, you have no way to tell exactly what. So let's be fully transparent and declare this hypothesis - and confirm it with a retest, using both definitions from MHRA - according to the usual rules (multiple determinations, etc.).
If your retest confirms your hypothesis, how to conclude your investigation? Let's be fully transparent again: your cause of the OOS was an unidentifiable lab error.
Don't fall into the trap of trying to define some blurry cause and then struggle to open a CAPA to prevent that cause. Be open about your results, and above that commit yourself that you are to identify the cause if this phenomenon occurs again.
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* let me add that this kind of situation may occur in simple tests with only little data to investigate - if it's about e.g., an LC or HS-GC test, normally you should have all your raw data and audit trail in your hands to determine the lab error cause.
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