I was what I think we would now call a "weeb" in my junior/senior year of high school, and had studied Japanese culture before making a short trip over there in the summer. One of the things I learned was that blowing your nose in public is seen as bad manners, and it really stuck with me. When you think about it, it is pretty gross to loudly blow snot into a tissue (bonus points for carrying a handkerchief!) in front of others, like (as an American) we'll just do this at the dinner table without batting an eye.

To this day, I try not to blow my nose in public places or in front of folks if I can avoid it, because it has grossed me out ever since learning how Japanese culture perceives it.