An air conditioning unit can freeze for a number of reasons. It might have low refrigerant, poor airflow, or a problem with one of several mechanical components. Some might call it "Pernicious Circulation." Dirty air filters block the flow of air. Without proper circulation, the evaporator coil can get too cold and freeze. If the drain pan is too dirty, too clogged, or simply too stagnant, it can hold too much water. When the system is running, the pan should be emptying out, but if it's full of freezing-cold water, is it really doing anything? When is a half-full drain pan a full drain pan? It's a good thing drain pans don't refrigerate the water inside them, and it's a good thing recycle bins don't freeze.