This scam was reported to occur in France, but it could certainly happen everywhere in the world.
As you are admiring the shops along the most popular streets in town, a well-dressed person posing as another tourist comes to you and asks for help.
She will inform you about a great deal at the store beside you – a Gucci jacket on sale (or any other brand name handbag, shoes, jewelry – pretty much anything). She will say that he already bought two and the cashier refuses to sell her any more because of a limit of two jackets per customer.
She hands you her money and, as a good person you are, you go in and buy her the jacket. The scam? The store is legitimate, the deal is real, but the cash is not.
How to avoid: when you are on the streets of any foreign country, never take cash from anybody. Not only can be counterfeit, but it could also be claimed back as a larger amount (see also the Money drop in Moscow scam).