If you think interview is really a measurement of one's professional ability, then like any measurement, it has measurement errors. To me, it seems that this person has some sort of interview anxiety because of previous bad interview experiences.. So the anxiety kind of feeds back onto itself. In this case, the interview can't really measure his ability accurately. But this brings another question... How valid and reliable are those interviews? I really have no idea about that since I have never had a tech interview. Just a question to think about when it comes to evaluating a measurement