There are so many laws that are passed that wind up having significant unintended consequences.
For example, companies were required to disclose the details of their CEO's compensation package. The intended consequence was that informed stockholders would drive companies pay to be more closely based upon performance. The unintended consequence was that executive pay soared since the informed CEO used the info in his negotiations with the board (and poor performers still get huge packages so it didn't fix the original problem).
Any other examples?