(Bloomberg) -- The most profitable short trade in town has been a gift that keeps giving. To be precise, it's had an eye-popping surge of more than 4,000% since the start of the Bull Run in 2009. That allure has been enticing investors to jump on the gravy train.

That gift — selling volatility — is so crowded now that when everyone reaches for the exit, it's bound not to end well.

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