What efficient markets are associated with which is wrong is that efficient markets mean that the price is always right – that the price is exactly the present value of all of the dividends and the earnings that are gonna come in the future and the price is perfectly right. That’s wrong. The price is never right. In fact, prices are always wrong. What’s right is that nobody knows for sure whether they’re too high or too low. It’s not that the prices are always right, it’s that it’s never clear that they are wrong…the market is very, very difficult to beat.
Source: The Upside of Academic Finance