The latest Freakonomics Radio podcast takes on something the called "The Cobra Effect." (Visit their web page for a full explanation; the short version involves a disastrous attempt by the Indian government to get rid of cobras.)
A finance professor who was visiting Bogotá, Colombia, noticed that his friends never picked him up in the same car two days in a row. Why? Vehicle use is restricted based on the last digit of the license plate. And the professor's friends had found a legal way to circumvent that rule: purchase another car. So a plan intended to decrease driving may actually have increased car ownership.
Listen to the story below. Or read the transcript here.