The most unrealistic thing about Harry Potter is that no teacher ever called Harry, James by accident, or that Ron never was called "Bill-, eh Charl-, no Per-, argh!"
Imagine Fred being called Percy by McGonagall accidentally and then he gets so offended that he refers to her by "Professor [insert any other name but McGonagall" for the rest of the year, costing Gryffindor a considerable amount of points one at a
time. From then on, she vows to just call them all Mr Weasley. Until Ginny comes along and she calls her Mr Weasley by accident and Ginny "accidentally' calls her Sir and it starts again.
It's lightly off-topic but also slightly relevant but I have long cherished this mental image of Professor Snape saying something snappish to Harry in just the wrong tone of voice and Harry absentmindedly, wearily, and completely accidentally responding with, "Yes, Aunt Petunia!"
