The law of unintended consequences is a frequently-observed phenomenon in which any action has results that are not part of the actor's purpose.
The superfluous consequences may or may not be foreseeable or even immediately observable and they may be beneficial, harmful or neutral in their impact. In the best-case scenario, an action produces both the desired results and unplanned benefits; in the worst-case scenario, however, the desired results fail to materialize and there are negative consequences that make the original problem worse.
Examples of the law of unintended consequences in play:
A company mandates security mechanisms, such as strong passwords or multifactor authentication, to protect sensitive data. However, because the new passwords are too difficult to remember or the procedures too cumbersome, users find ways to circumvent the mechanisms, such as writing passwords on sticky notes on the monitor.
In the United States, the Patriot Act expanded the power of law enforcement and government agencies to monitor and intercept the data of private citizens. One unintended consequence was a reluctance of companies and individuals to allow any of their data to be stored in the U.S.
As machine-to-machine (M2M) communications and the Internet of Things (IoT) develop, an increasing number of devices have the capacity to transmit data over a network. However, these devices are often things that have not traditionally had any ability to communicate and as such have no security mechanisms in place to protect them. An unintended consequence is security attacks on IoT devices, which have included a light bulb hack.
Factors that reduce the likelihood of unintended consequences include an understanding of the systems involved, careful planning and an attention to detail during execution. In recent years, the law of unintended consequences is often evoked in reference to complex systems, which by definition cannot be fully understood. As a result, any action that involves a complex system is certain to have unintended consequences.
In this TED talk, historian Edward Tenner discusses the gap between our ability to innovate and our ability to foresee the consequences: