We are three seasoned travelers who have been to paris many times.
However, we lost 7 days pass cash is 20 euro each because a so called well dressed man offered to use his credit card to charge the pass. It was never invalidated and we lost the cash given to him. Never trust a stranger any time any where. Paris metro has the most complicated system for tourists to purchase metro pass. The procedure included purchasing a coupon from a machine, then take the coupon to the information booth to exchange for a pass, then go to the machine to fill the pass for aaxim of 7 day pass. Some machine takes cash only and often exact change. The information receptionist was not helpful. We were of too much bother to her, we were supposed to know all the ins and outs.
We later also got pickpocketed by a group of young women. They just crowd in to one of us and picked the cash out of even money belts. Amazingly fast and skillful.
The lesson is :
Study the metro card purchase thoroughly and carefully before you go to the metro and prepare cash or credit card.
Do not go into crowded cars of the metro. Pick off hours to travel on metro.
We ended up using the bus for the rest of our stay. Slow but less chance of pickpockets.