Today we had a sudden cold snap here in western Pennsylvania, with the temperature dropping 30 degrees F. I was walking through a white field that had been green yesterday, looking at daffodils poking up through the snow and feeling irritated that they'd probably die. It occurred to me that, if we could control the weather, people would probably vote for a smooth transition from winter to summer, and this would wreak some unforeseen environmental catastrophe, because it would suddenly make most survival strategies reliably sub-optimal.
This is typical environmentalist thinking: Whenever you see something in the environment that you don't like, stop and step back before trying to change it. Trust nature that there's some reason it is that way. Interfere as little as possible.
The classic example is forest fires. Our national park service used to try to stop all forest fires. This policy changed in the 1960s for several reasons, including the observation that no new Sequoia saplings had sprouted since the beginning of fire suppression in the 19th century. Fire is dangerous, destructive, and necessary.
Leftist are social farmers; conservatives are social environmentalists
Most leftists view their own society  the same way farmers view the environment. They believe we can all agree on what crops to grow (what social values to have ), and we should clear out the stumps of old institutions, plant the seeds of the new, create the right rationally-informed infrastructure of irrigation, fertilization, crop rotation, etc., and pull out and burn the weeds of nostalgia, counter-revolution, and the bourgeoisie, and we will have a modern, rational society.
Conservatives view their own society the way environmentalists view the environment: as a complex organism best not lightly tampered with. They're skeptical of the ability of new policies to do what they're supposed to do, especially a whole bunch of new policies all enacted at once.
So, many liberals and many conservatives understand the law of unintended consequences (LUC), but the contexts they apply it in are disjoint.
If we broadly stereotype conservative Americans as Christian, and Christians as not believing in evolution, we can explain why liberals would be more likely to apply LUC to the environment. An evolved system is complex and dynamic, and can lose its stability. A created system is presumed to be static and always stable, so Christians don't consider LUC to be an issue with respect to the environment.
If we stereotype progressives as social constructivists, we get the same result for society. Progressive philosophy derives from post-modernist philosophy, which, to a first approximation, is medieval philosophy and physics with a global s/God/humans/ run on it. It focuses not on the creation of the world, but on the creation of society.
Marxism, which is the basis for academic progressive thought today, takes as axiomatic that economic relationships are the base that determines all else. Leftists therefore believe that economic relationships don't depend on anything else, and can be restructured arbitrarily. There can be no avalanche of changes resulting from restructuring the base, because effects on the superstructure cannot cause changes in the base. Therefore, to a Marxist, the LUC is not relevant to human societies, which are simple in a way that the environment is not.
 Most leftists view other societies the way environmentalists view the environment. I recall, for example, Edward Fischer's Teaching Company lecture series on anthropology, "Peoples & Cultures of the World," which begins by explaining that it is important not to judge other cultures for institutionalizing wife-beating, community gang rape, mandated homosexual rape of boys, genital mutilation, wife-burning, untouchable castes, and so on--and then concluded with a series of lectures giving a Marxist critique of our own culture, from an implicit objective standpoint which for some reason can be used to judge only our culture.
 The truth of this statement is obscured by the radical post-modernist wing of leftist politics, which claims to be relativist, and that its goal is only to avoid privileging any one position over any other. In practice, this goal is itself as absolutist as any divine revelation. In any case, post-modernists cannot claim to represent or even be progressives, as rejection of the notion of progress is the first axiom of post-modernism.