Sirius Black high-fiving Lily Evans every time she raises her hand in class.
Sirius asking James to high five her when he is too sick to come to class.
Remus Lupin doing it with a sigh and an immediate apology when neither Sirius or James is available.
Peter being overly enthusiastic about being asked to do it, and slapping Lily in the face by accident.
When people in movies run directly away from the train / boulder / truck / etc instead of just like, taking two steps to the side of it
OH NO A GIGANTIC TREE FALLING OVER *runs away directly along its length*
Bucky knows what’s up