I had thought that this was a third world thing. That sort of thing surely couldn't happen in Europe, could it?
So imagine my delight when, reading through the Lonely Planet Spain I found under the "Dangers and Annoyances" section on page 118 just after the bit about pickpocketing and prostitution (that's dangerous and annoying?), the following:
Be on the lookout for classic scams like the old bird-dropping trick: a thief will drop something brown on you then help you clean it off as he helps himself to your wallet."
FANTASTIC!
I won't rest until I get the dogshit treatment in Madrid!