It's inevitable that the market will have some inefficiencies, the question is: how can you tell whether you found one? Mark Cuban claims he got rich because he found stock price anomalies in a business he was knowledgeable about, namely network equipment [1]. But he also warns that every stock trade has a sucker involved, and how do you know that you're not the sucker (i.e. the other party knows the stock is going to go down and that's why they're selling)?

1. http://blogmaverick.com/2013/01/10/the-stock-market-2/