I've seen many discussion board comments like this: "Do you have the required expertise to talk about the Narmada dam issue?", "I've worked in the election commission for 15 years, what rights do you have to question me on election fraud in rural India?", "I have a masters degree in journalism, don't teach me how to report riots"... I understand this style of argument because I've been there myself. This is mostly the result of reaching a point where they realize that they cannot satisfactorily progress (based on logic/facts/etc) a discussion because of their half-baked understanding of the issue or a realization that the other guy has brought valid points to the table that cannot be refuted and instead of conceding the discussion in an honest manner they hide behind their credentials.