Which "law" of supply and demand are you thinking of? To my knowledge, there is no single such law, just a general rule of thumb. Simple mathmatical models in economy, in which "laws" can be proven often don't predict the real markets all that well.
I explicitly did not call it a law, and even explained why it cannot be called such, so I have no clue why you are calling me out on that.
Regardless, I take exception to the claim that they "often don't predict the real markets all that well". They can predict real markets very well in most scenarios. Most theoretical-basis predictions of price ceilings and floors (such as rent control or minimum wages) are perfectly sound, but there is plenty of ambiguity in the results that critics seem to misinterpret as proof of the failure of Supply and Demand to predict outcomes.
Price ceilings tend to result in shortages, but occasionally result in no effect at all. But that isn't enough evidence to say that Supply and Demand doesn't apply to this scenario. In order to disprove the applicability of Supply and Demand, you would have to show that a price ceiling increased the quantity supplied. Such scenarios are as rare as black swans.
Say's Law certainly isn't the only economic proposition claimed to be a "law", though it is arguably the most misunderstood and most widely regarded as false of all the "laws"...
