1. Fail to say bonjour.
French politeness is predicated on the use of formulations. You don’t need to talk extensively (in fact, you shouldn’t unless you know the person), but you must always say “Bonjour”. When you walk into a tiny boutique and you are only interested in looking; when you arrive at the office with a hangover and no desire to speak to anyone; when you ask for directions on the street; when you buy a bus ticket; and yes, when you walk into the waiting room at the dentist’s office. In fact, nearly every conversation should start with “Bonjour” or “Salut, ça va?” if you don’t want to develop a reputation for being antisocial and mal élevé.