I agree with Crhis but I think I wasn’t clear with my question.
I don’t want to identify failing cases because I want to handle them differently.
In my scenario all functions should return {:ok, value} or {:error, reason} and that else clause is irrelevant.
But one of the functions does not conform to specification and returns something different which I’d like to fix. I encounter that scenario fairly often and each time it is a pain 
