One other example of unintended consequences is the $20 raise we all got by not paying all our Social Security taxes to buy votes in the 2010 election. It didn't buy many votes (thank God), but it cost Social Security $16 billion dollars, and shortened its life expectancy by two years.
One more example: The Fed has kept interest rates near zero for years to stimulate the economy. It hasnt done much good, but the unintended consequence of this is that people who worked hard and saved all their lives to live comfortably in their retirement are getting no interest on these savings. Thanks Fed.