Hysterically funny post a while back at Evolgen giving some humourous impressions of the kind of people that end up at faculty or other seminars. We've all been to seminars and talks like this, where the head crank seizes the floor, in fact we may have even been some of these audience members.
The grand advertiser: No matter how unrelated, insignificant, and uninteresting this guy's research is, he'll find a way to link it to the topic of the seminar. And no one will give a shit. The speaker will pay lip-service to the question, but we've already died a little inside because the grand advertiser has wasted our time advertising the crap his lab studies.
And many more are suggested in the comments.
Any library-specific seminar attendees? The Piner for the Good Old Days of Card Catalogues comes to mind, but those tend to be more mythical than actual, in my experience.