Ag subsidies are a case in point.
Farmers don't farm in order to provide our nation with a stable food supply. They do it to earn a profit. If the free market alone is making the decisions, and the decison happens to be that some farmers can't even make enough to meet their expenses, then packing it in and heading off to find other work may not merely be the rational course of action for them; it may be the only choice. If the free market then decides that some time is required to allow this fairly inflicted economic wound to heal, and that that means allowing what was once a productive farm to sit idle as an entry in the ledgers of some bank, then that's how it's going to be, and if the remaining farmers can't pull up the slack, then the nation's food supply can go jump off a bridge. Government ag subsidies came into being because it was noticed that that very thing did indeed happen rather often, and that it was not only the farmers who suffered, but everyone else as well.
Without a doubt, that system has been subject to much abuse over the years, and it's tempting to look for a simple solution like just doing away with subsidies, but the problem with simple, heavy-handed solutions to complex problems is that they usually don't work very well.