I was amused to read this article at the Foundation for Economic Education's web site.
In colonial India, Delhi suffered a proliferation of cobras, which was a problem very clearly in need of a solution given the sorts of things that cobras bring, like death. To cut the number of cobras slithering through the city, the local government placed a bounty on them. This seemed like a perfectly reasonable solution. The bounty was generous enough that many people took up cobra hunting, which led exactly to the desired outcome: The cobra population decreased. And that’s where things get interesting.
As the cobra population fell and it became harder to find cobras in the wild, people became rather entrepreneurial. They started raising cobras in their homes, which they would then kill to collect the bounty as before. This led to a new problem: Local authorities realized that there were very few cobras evident in the city, but they nonetheless were still paying the bounty to the same degree as before. City officials did a reasonable thing: They canceled the bounty. In response, the people raising cobras in their homes also did a reasonable thing: They released all of their now-valueless cobras back into the streets. Who wants a house full of cobras?
In the end, Delhi had a bigger cobra problem after the bounty ended than it had before it began. The unintended consequence of the cobra eradication plan was an increase in the number of cobras in the streets. This case has become the exemplar of when an attempt to solve a problem ends up exacerbating the very problem that rule-makers intended to fix.
. . .
Unintended consequences arise every time an authority imposes its will on people. Seat belt and airbag laws make it less safe to be a pedestrian or cyclist by making it safer for drivers to be less cautious. Payday lending laws, intended to protect low-income borrowers from high lending rates, make it more expensive for low-income borrowers to borrow by forcing them into even more expensive alternatives.
Requirements that corporations publicize how much they pay their CEOs in order to encourage stockholders to reduce CEO pay resulted in lesser-paid CEOs demanding more pay. Three-strikes laws, intended to reduce crime, increase police fatalities by giving two-time criminals a greater incentive to evade or even fight the police. The Americans With Disabilities Act gives employers an incentive to discriminate against the disabled by not hiring them in the first place so as to avoid potential ADA claims. Electrician licensing requirements can increase the incidence of injury due to faulty electrical work by reducing the supply of electricians, thereby encouraging homeowners to do their own electrical work.
There's more at the link.
I hadn't heard of the cobra example before, but the unintended consequences of well-intended laws, rules and regulations are legion. I'd love to have some way of adding up all the costs of such unintended consequences throughout history, and seeing the size of the total. I suspect it'd be astronomical!