There isn't much that people can say that bothers me, but when someone you're having a conversation with finds out you're a diabetic (I don't know about you, but I don't publically announce this information usually) and then spurs up a conversation like they're a know-it-all on the subject only to give erroneous or innaccurate information, that just seems to get under my skin.
I can't blame people for not knowing about something they don't live with themselves, but please, please, please save yourself the embarrassment that follows when you think you know something about it but you really don't.
Although, on the flip side of this - sometimes I can do nothing but laugh at it because some of the information they claim to know is so wildly bizarre. hah. ;-)