Behave, baby. Wikipedia’s list of French faux pas. Here’s one: “Like in many languages featuring a T-V distinction, addressing people with the familiar “tu” (like in Middle English thou) when they should be addressed with “vous” (_you_) is seen as derogatory, insulting, or even aggressive. Conversely, addressing familiars with “vous” is considered snobbish and introduces distance.”
Via kottke.