Here's a really good scam which I read about in Stephen Jory's book about currency forgery, Funny Money. It turns out that many of their forged £20 notes were sold to taxi drivers, who used them to defraud their customers.
The way it works is as follows:
Customer goes on taxi ride, offers £20 note in payment.
Taxi driver takes note, swaps it in his hand with the forged £20, and says ``Haven't you got anything smaller?''
If the customer replies ``No,'' the driver hands back the fake £20 and says ``Don't worry mate, this one's on me.''
If the customer offers another note in payment, the taxi driver hands back the fake £20 and takes more of the customer's real money.
Cunning, but it hasn't improved my opinion of taxi drivers any.