Exchange-traded funds have taken the investing world by storm, having made it easy for millions of investors to get exposure to the overall stock market as well as niche investment areas. But lately, we've seen one big flaw emerge with ETFs that investors need to be aware of.

In the following video, Fool contributor Dan Caplinger notes that recently, several ETFs have shown big disparities between their share prices and the underlying value of the assets. Dan points out that while this is fairly common among international ETFs, where foreign markets aren't always open when U.S. exchanges are trading, the phenomenon has spread to U.S. bond market ETFs. Dan concludes that a lack of liquidity is always a concern with ETFs, and so you really need to look at the underlying assets before you buy ETF shares.