Here's one price control I can agree with.

Price controls never control prices in the way the controllers wish.  Instead, they 'control' quantities demanded, either increasing demand (with a price cap) or diminishing it (with a floor), but in neither case can markets clear. Short-term net result is market chaos; longer-term result is withered markets.

This is generally a bad thing.

But there might be one exception. And the European Union may have just found it:

Any price cap increases demand while reducing producers' ability to meet the supply demanded. Short-term result is undersupplied markets, black markets, and reduced quality. 

But what about when it's only a notional market anyway?

Notes David Turver:

"Capping carbon prices is a small step in the right direction, but scrapping the ETS altogether would be better because paying carbon taxes to the Government won't change the weather."