My pet peeve is not ignorant people, its people who are ignorant but who *think* they know what they are talking about.
Like management where i work, who are not doctors but who are scared to have reported injuries, so they tell employees who are trying to report an injury that they are 'fine', its just a such and such, just go home and sleep it off, as if they have an M.D. Telling people hit in the head to just go home and sleep it off? Are they crazy? You don't need that insulin you left in your car right now, it can wait till after work, now get back to work!
Or people who claim that the law says this or that, having never read the law in question, nor any understanding of how the law works.
Or, as an atheist, when arguing (in a friendly manner) w/a christian, who brings up the bible to defend their position and they get their facts way wrong (I can't tell you how many christians there are in my neck of the woods who never read the bible. Fine, don't read it, believe completely based on faith, fine, just say so, but don't go pretending to be an expert on something you haven't read.)
I mean when i claim something i am unsure about, i say so. I say something like 'I could be wrong, but its my impression that such and such is the case'. I make it clear up front that this is what I believe and why, but that I'm not entirely certain that my info is right or that I have all the right info, and i accomplish all that w/that one simple addition to my statement. Then i get my info confirmed or i get enlightened as to where i'm wrong and come out knowing more than I came in with.
I think its about ego. People just can't admit, even to themselves, that their knowledge has limits (even though for all of us, our knowledge has limits, none of us are all-knowing). Its the old saying, 'a little bit of knowledge is a dangerous thing'. The people who know a little, who then go around thinking they are an expert, unintentionally misleading others into following their advice.