Unintended consequences

Unintended consequences are the surprising results of an action or decision.
When an outcome is not anticipated or foreseen, the situation at the end of a process may be different from what was intended. In other words, purposeful action may create consequences which are 
This concept is one of the building blocks of economics. The term was coined and popularised in the 20th century. Its use has expanded into a range of contexts.
History.
The idea of unintended consequences dates back at least to the 18th century when Adam Smith wrote about men who are "led by an invisible hand to promote an end which was no part of his invention".
American sociologist Robert K. Merton invented the phrase in 1936.
Types.
There are three types of unintended consequences:
Causes.
Possible causes of unanticipated consequences include
Select examples.
There are examples and kinds of unanticipated consequences:
Cheaper and easier travel.
Cheaper travel, mostly in the form of air transport, is hugely beneficial to the economy by increasing tourism into a country. It accounts for 30% of the world's trade of services, and 6% of overall exports of goods and services. It also creates opportunities for employment in the services needed for tourism.
With increased tourism comes a variety of unlooked-for problems, such as increased illegal immigration. Perhaps even more serious is that returning tourists may bring back parasites and diseases which are not usual in the home country. Furthermore, many of these organisms are now resistant to most treatment.