The 2010 flash crash is a reminder that market prices only look solid as long as someone is willing to quote them — bad news for anyone who assumes they can always sell.

• On May 6, 2010, the firms that stand ready to buy and sell stepped away from the market for a few minutes.
• With nobody quoting one side, well-known stocks briefly traded for a penny.
• Those prices had nothing to do with what the companies were worth — there were simply no buyers at that moment.
• The cheap, easy trading people take for granted depends entirely on those firms staying in the market.

Outlook: Liquidity can vanish again in seconds, so the tight prices investors expect are never guaranteed in a panic.