It was a year ago this week that a flash crash sent ETFs into uncharted territory. Since that time asset managers and market exchanges have worked to make sure one of the most volatile days of market trading ever never happens again.

On Monday morning August 24, 2015, concerns about the Chinese economy caused volatility to surge across a number of blue-chip stocks. The Dow Jones Industrial Average dropped over 1,000 points at one point before closing down 588 points, or 3.57% for the day. The S&P 500 finished the day 4% lower.

As a result of the unusually high volume, there were an abnormally high 1,278 trading halts. ETFs that typically trade essentially in line with their net asset values (NAV) were negatively impacted as trading in some of their stock holdings was halted. For example, the iShares Core S&P 500 Index ETF (IVV) traded at a 22% discount to its NAV within the first three minutes of trading.

"It was disorienting for investors when stocks invested in their ETFs were halted," said Rosenbluth. "The limited amount of information had people scrambling."

Since then, a broad group of market participants -- including the ETF industry that collectively manages $2.4 trillion in assets and is the beneficiary of 30% of daily trading volume -- has been eager for the exchanges to work together to put in places rules to limit the likelihood of a repeat occurrence, according to Rosenbluth.

In March 2016, SSGA Funds Management (STT) worked with other asset managers including BlackRock (BLK) , Charles Schwab (SCHW) , Franklin Templeton (BEN) , JP Morgan (JPM) , Vanguard Group, and Wellington Management on proposed remedies. And three major exchange groups, Bats Global Market, Nasdaq and the New York Stock Exchange, announced plans to increase resiliency during times of extreme volatility.

The scrutiny has already paid off. There were only 68 U.S. trading halts triggered across the market on June 24, the day Brexit results were announced, far fewer than what occurred on August 24 last year, according to Bats Global Markets.

While the flash crash may have been unnerving at the time, Rosenbluth said the ETF industry has not suffered any long-term damage as evidenced by the significant inflows over the past year.

"People remain confident in the products," said Rosenbluth. "But they learned a good lesson not to buy or sell at the open. It pays to wait until trading gets under way before placing an order."

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