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WARNING! Remember to keep your ticket until you are back on the street. If you get caught in a ticket check, some will allow you to go back to find the validated ticket you threw away, but most will not. And if you are caught without a ticket, it is a €75 fee. Per person without a ticket! Of course this only applies to paper tickets, not Navigo cards.
@SamChats, yes, it is likely that the officer will ask for your flight itinerary. It is wise to keep such papers (or copies of them) with you during your stay in the US, especially if you travel to states in the Southern US regions. Just beware of pickpockets (thieves).
