I tend to not think of someone as a know-it-all if I only know them in one area of knowledge. I tend to take the assumption that maybe yeah, maybe they DO know that. If it seems like they're like that in alllll subject areas... then I just take what they say with a grain of salt, let them blather on, and do my own thing. Unless what they say is potentially dangerous, sexist, racist, etc... in which case I'll speak up.

I tend to feel a lot of empathy in situations like the one you were in - I'd most likely jump to the thought that maybe they were just really nervous and over-compensated, and that then others were really nervous and went along with it out of fear of seeming like an idiot. If there were actual experts in the field around, I'd just refer to what they were saying.