Vaguely related: I had a friend in college who made grotesque faces as a greeting if we spotted each other across the room

One time I was walking along with my boss, the head of the school library, and I saw my friend coming up the stairs. She made this scowly sneering face, baring her teeth--before realizing who I was walking with, and who thought the face was being directed at her. Took me a while to stop laughing before I could properly explain to the bewildered librarian that no, it was ok, she was just saying hi to me :'D