Markets will be permanently efficient when investors are permanently objective and unemotional. In other words, never.—-Howard Marks, Oaktree Capital
This quotation is taken from a much longer think-piece about the role of luck in investing that I first saw on Advisor Perspectives. Mr. Marks points out that while markets are often structurally fairly efficient, they are often quite inefficient on a cyclical basis when investors freak out. Highly recommended reading.