Two Canadian pennies, upside down.
This was at a restaurant in Buffalo that my friends and I back in our post-college years occasionally frequented. The waitress, who we never saw before, was completely, totally silent with us -- never uttering a word -- but she was friendly with other customers. She also took much longer to take our orders and deliver our food than other tables she had. We have no idea why; we weren't a group of ponytailed geeks loudly reciting lines from Monty Python skits, or frat boys just being obnoxious, but rather didn't look or behave any differently than anyone else in the place.