In Friday's Wall Street Journal Fund Track, Ian Salisbury examined the 300 recent trades in 10 lightly traded ETFs, and found that some have fallen short of their promise to let investors trade baskets of stocks at predictable prices. In a couple of instances, trades were off by more than 10 percent, according to Salisbury. The average price investors received, meanwhile, was a little more than 1 percent off.

Closed-end funds, a relative of ETFs, have historically traded about 4 percent below their holdings' value. 

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