I think that, as you said, examples of policies having unintended consequences abound. Furthermore, there are cases where a policy could in fact induce opposite effects to those originally intended. A great example, in my view, has to do with police presence and its effect on public safety. A number of papers suggest increased police presence improves citizen’s safety perceptions in contexts where there’s trust in forces of the law, and could hamper such perceptions if citizens suspect police officers. I’ll leave a related link below.