It grinds my gears when:
Someone accuses me of not having the decency to do a little research on a claim, when the person hasn't bothered to read that I supplied the bibliography entries to two professional journals and one doctor's book on traditional diets, and comes up with an "Eating Well" web article that exactly proves the point I made of American adaptations of the Mediterranean Diet not being as healthy as a traditional Cretan diet, all the while claiming the American adaptation of the Mediterranean Diet is indeed healthier than the traditional Cretan diet.
It's like a mommy blogger accusing someone of losing weight dangerously, when the reducer follows dietetic guidelines from a doctor who runs an intensive dietary management clinic for diabetics, because the mommy blogger can't accept that losing adipose fat doesn't require exercise. The mommy blogger is convinced that the caloric restriction must come from energy expenditure, and not a combination of reduced caloric intake and elimination of foods that promote inflammation. A woman could never lose weight that way, only through squats and purchase of Lululemon exercisewear, thinks the mommy blogger!
Or, a red-state small-town morning newscast weather person arguing against climate change with Bill Nye.
"Um, even though this is on the internet where we could both look up sources I'm going to just say you are wrong and uninformed rather than insist you show me your sources, because then I can at least provoke you into showing me your sources. I'd really hate for you to be calm while you're showing me how I'm wrong or uninformed. "