The first and most complete analysis of the concept of unintended consequences was done in 1936 by the American sociologist Robert K. Merton. He identified five sources of unanticipated consequences. Ignorance, error, imperious immediacy of interest, basic values requiring prohibit certain actions and self-defeating prediction or the self-fulfilling prophecy.
In medicine, most drugs have unintended consequences, or side effects, associated with their use. However, some are beneficial. For instance, Aspirin, a pain reliever, is also an anticoagulant that can help prevent heart attacks and reduce the severity and damage from thrombotic strokes. Famously, the drug Viagra was developed to lower blood pressure, with its main current use being discovered as a side effect in clinical trials.
The law of unintended consequences is at work always and everywhere. The law of unintended consequences provides the basis for many criticisms of government programs. As the critics see it, unintended consequences can add so much to the costs of some programs that they make the programs unwise even if they achieve their stated goals. A policy that protects one industry from foreign competition makes it harder for another industry to compete with imports.
If cars, trucks and buses start driving themselves, people who earn their living from driving these vehicles will suddenly find themselves out of a job. Loss of driver jobs in USA alone would be 4 million - which is more than jobs lost during 2008 Great Recession. Account for all the supervisory, management and support staff for these driving jobs and that number could double. Many of these workers are classified as low-skilled workers and it will be difficult for such unemployed workers to quickly find new work, and the cost of re-training them could be high. Another interesting consequence is that after a few generations, very few people will even know how to drive a car anymore.
An example of unintended consequence: Social Security has helped alleviate poverty among senior citizens. It has carried a cost that goes beyond the payroll taxes levied on workers and employers. Today’s workers save less for their old age because they know they will receive Social Security checks when they retire. It means that less savings are available, less investment takes place, and the economy and wages grow more slowly than they would without Social Security.
An unintended consequence of China eradicating sparrows costed 20 million lives in late 1950's: During the late 50s and early 60s, communist China's Mao Zedong undertook a benign ecological program to kill sparrows which ate the grain of the peasants. Mao knew nothing about animals. He didn't want to discuss his plan or listen to experts. He just decided that the 'four pests' should be killed. The masses of China were mobilized to eradicate the birds that resulted in the near-extinction of the sparrows in China. By killing sparrows, they eliminated the primary enemy predator of the locust. Without any natural predator the locust thrived. Grain production in most rural areas collapsed and a massive famine began. The resulting massive famine in China killed an estimated 20 million people.
The worst unintended consequences are being faced by Indians consequent to senseless demonetization by Modi during Nov 2016. He did it just Mao's way - a quack advised & secretly planned in a most undemocratic and autocratic way surprising the whole world. The consequences are enormous including loss of lives of over 100 people. The brunt was borne by poorest, as usual. Read the opinions. While achieving none of its stated objectives, unintended consequences are growth slowed, informal economy brought to stand still, millions lost livelihoods, agriculture destroyed, banking sector ruined and so on. It was aptly described as most naive, least thought through policy decisions ever and a massive man-made disaster. Steve Forbes described it as "India's government perpetrated an unprecedented act of demonetization, that is not only damaging its economy and threatening destitution to countless millions of its already poor citizens but also breathtaking in its immorality...What India has done is commit a massive theft of people's property without even the pretense of due process, a shocking move for a democratically elected government." A vicious cycle has been set into motion by Modi and unpredictable problems and unintended consequences are bound to surface incessantly. It may take few years to come back to pre-Nov 2016 levels of economy. However Modi preferred keep Indians in denial by not answering Parliament, fudging economy's data, and publicizing its failure as successful and is the beginning of relentless fight against corruption. Ironically, Modi too is corrupt in many different ways, but much less compared to Congress and his Gujarat CM days were knows as Mr. 3%. His stature doesn't august well in the company of Adani's & Ambani's as well.