The Law of Unintended Consequences is best demonstrated when philosophical liberals decide to solve problems through legislation. Liberals like legislation. It lets them control others for causes that the liberals themselves feel good about. Legislative solutions are simple; you need only get a law passed rather than changing public opinion. It produces concrete changes, which the liberals can point to and also feel good about. And it lets liberals have an emotional catharsis of rage towards anyone who opposes their legislative agenda. Unfortunately there are unintended consequences to mucking about with complex systems.
Complex systems are... complex. It is difficult to understand the full range of interactions in complex systems. We are just starting to learn this about nature. Biological systems are complex, and the way they interrelate can cause problems when we interfere. For example, if there is a particularly difficult winter, we might be tempted to feed wild deer. But this has an effect on the number of predators who survive the winter.
Yes, we're starting to understand this about nature. But unfortunately liberals will never understand this about economics. Passing laws to regulate the economy is simple, and appeals to liberals. But the economy is a complex system, and if you muck about with it there are consequences. And it gets even worse when you mix social interactions with the economy.
A case in point is ethanol. Liberals felt it was good for the environment to include ethanol in gasoline. But not content to let people make environmentally responsible decisions for themselves, liberals convinced themselves that the best thing to do was to legislate 10% ethanol in gasoline. Which let the liberal feel good about themselves. And then unintended consequences took over. Today, 30% of North American corn production is going towards producing ethanol. Instead of feeding human beings. Which means the prices go up. And not everyone can afford to pay, which means a lot of people are worried about hunger on a worldwide basis (and yes, there are other crops and other issues which are affecting the prices of those crops).
So what's the answer? Repeal the laws that create the problem? No, that's not the liberal way. The liberal way is to pass more laws, with more government control. Which sounds reasonable and compassionate, until the next time the law of unintended consequences strikes.