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This is a variation on the Infinite Monkeys Theorem.

Take 1024 monkeys and have them randomly choose between buying or selling a stock. Ignore the (approximately) 512 that were wrong, and have the remaining ones choose between buying or selling a stock. Keep going until at the end you have a very rich monkey, who basically made his money just by randomly guessing and being lucky. Now all the other monkeys think he's an investment genius.

Of course, 1024 monkeys isn't much of a sample size, so if instead you had one monkey for each human being on the planet, you could find a monkey with a much longer series of successful investment decisions (and a much larger investment portfolio to "prove" he's a genius).