Off the top of my head, one of the best examples of "the law of unintended consequences" occurred in those States that made "texting while driving" against the law (with a hefty fine)---in an effort to decrease auto accidents.

The initial result was that auto accidents increased.

Further investigation revealed that "text-addicts", who used to position their devices on top of the steering wheel while driving, began holding it in their laps or under the top of the dashboard, where it could not be seen by passing police. Consequently, they had to take their eyes off the road more often, and caused more accidents.