ETF - Exchange Traded Fund - "A marketable security that tracks an index, commodity, bonds, or a basket of assets like an index fund."
In the last decade, ETF's have grown in popularity more than any other investment.
The graph below shows their explosive growth.
Q2 2006 Value: $17,565,000,000,000 - Nearly 18 trillion USD$ in ETFs
10 years later +$384,120,000,000,000 (Yearly growth rate - 219%)
Q2 2016 Value: $401,685,000,000 - Nearly 402 trillion USD$ in ETFs
An issue that could be amplified by ETFs larger market presence is that they can fundamentally fail.
Flash Crash's 2010 & 2015
Flash Crash Wiki
In the "Flash Crashes" of 2010 and 2015, ETFs became unhinged from their underlying "value". Both instances were partially caused by HFT (High Frequency Trading).
"Bids on dozens of ETFs, and other stocks fell as low as a penny a share" -WSJ
(^talking about 2015)
In Complex Adaptive System Theory, a Cascading Failure is when the failure of a part causes the failure of successive parts. (Domino Effect)
Both flash crashes were cascading failures in financial markets. Regulatory measures taken to prevent another ETF issue are insufficient.
Is not responsible for your investments.