(Note: needs better title)
This is the thing where someone insults your friend, or your car, or... whatever. And they say the same thing you say and/or think EVERY DAY, but... that's only okay when they say it.
Named for Mean Girls. The Trope Codifier for this troper was in Harry Potter and the Prisoner of Azkaban, where Snape takes points off Hermione for 'being an insufferable know-it-all'. The narration notes: 'Hermione went very red, put down her hand, and stared at the floor with her eyes full of tears. It was a mark of how much the class loathed Snape that they were all glaring at him, because every one of them had called Hermione a know-it-all at least once, and Ron, who told Hermione she was a know-it-all at least twice a week, said loudly, "You asked us a question and she knows the answer! Why ask if you don't want to be told?"'