Interesting, could you describe one inefficiency that was exploited in the past? I could imagine buying/selling due to spreads between exchanges but is there another not as obvious example?
Like sports gambling, a lot of the financial products we trade are obviously built by humans using rules, and arbitraging the intrinsic rules and regulations around said products. Think about Forex trading where you convert currency into currency. One of the key strategies is to find and identify brief negative cycles, for example, in the hope that converting US Dollars to Euros to Yen back to US Dollars leaves you with more dollars than you started out with.