How about the guy who's in the wrong forum? e.g-someone who thinks he's a very wise amateur philosopher, so he joins a philosophy forum and tries to sound intelligent around people who actually are. Or someone who has successfully kept a cactus alive for a month, so he joins a gardeners forum and imparts his wisdom every chance he gets. And I don't even want to discuss the high school biology student who joins the brain surgeons forum.
How about the guy who complains about people not belonging in a forum, because they don't know enough about the subject?