The Rick Steves web site has a collection of tourist scam stories. Some are pretty clever.
My husband and I arrived at Paris’ Gare du Nord train station in the early evening, and proceeded to read the map to find out how to get to our hotel via the Métro. One guy came up and advised us to buy tickets from the ticket machine. When we were at the machine trying to read the French, another guy came out and “helped” us to buy tickets. Later, what was supposed to be a three-day ticket turned out to be a one-way, single-use ticket. We paid him €48, the price shown on the ticket machine, but he must have cancelled the transaction and bought us the single-trip ticket instead.
(hat tip: Schneier on Security)