Well, the sad truth is that a lot of _interviewers_ actually suck at interviewing, so sometimes an interview devolves into a bewildering game of what this particular person wants to hear today based on their mood, especially with the open-ended questions. It is not always possible to suffer through this process politely and constructively.

If the candidate is exposing flaws in your assessment and making you think that should be a positive signal, not a negative one. Unfortunately people have egos and generally don't take kindly to situations like that.