Two quick and excellent examples of the law of unintended consequences, both courtesy of the National Bureau of Economic Research:
1. Laws cracking down on cigarette smoking by banning smoking in bars and restaurants and by increasing taxes on cigarettes have contributed to an increase in obesity. Reports the Atlantic: "Smokers are less likely to be obese. And the declining use of cigarettes across the country -- due to both tightening pocketbooks and new laws (thanks, Mayor Bloomberg) -- accounts for a bigger increase in the obesity rate in the U.S. than any other factor, according to paper authors Charles L. Baum and Shin-Yi Chou, who have both written with some frequency on the economics of obesity."
2. A paper by researchers Susan Busch (Yale), Ellen Meara (Dartmouth), and Ezra Golberstein (University of Minnesota) found that after the Food and Drug Administration decided in October 2004 to require "black box" warnings on antidepressant packages about the supposed risks of pediatric antidepressant use, pediatric antidepressant use fell 20% to 30%. Depression and suicide both rose in response. Grade point averages sank and delinquency behavior (fighting, stealing) and substance abuse rose among the newly unmedicated depressed teenagers. The paper says that "the unintended consequences of policies designed to protect consumer safety extend well beyond the clinical considerations that typically contribute to decisions about the costs and benefits of regulatory actions."