The latest big worry to hit markets is an unusual one: calm. With stock prices high and various gauges of risk low, investors appear to have thrown caution to the wind.

That isn't entirely true, though. Exchange-traded notes that profit handsomely from market-shaking events have boomed since the financial crisis. But they have two big shortcomings: They may not work as designed in another financial crisis since their value depends on the bank backing them. And due to the way the products work, anyone holding these for the long term will inevitably see their value erode.