Be careful of pickpockets on the RER B to Paris city center from Paris-Charles de Gaulle airport. Thieves are known to take the train, act like they don't know where they are going so they can distract you and steal your possessions. If you see someone suspicious, walk away from them.
Also, be careful in the large stations such as Gare du Nord and Châtelet/Châtelet Les Halles. Only take the possessions you need and keep your bags close to you. This goes for any city you visit, not just Paris, France.