The Dow Jones Industrial Average slumped nearly 1,000 points in a matter of minutes in the flash crash of 2010, sending traders into a panic and inciting scrutiny of the U.S. equities markets that's still being felt four years later.
The May 6, 2010, crash was initially blamed on a "fat-finger" error made at Citigroup—a theory that was later shot down and ultimately attributed to investment firm Waddell & Reed. But in addition to that trading error, a number of possible reasons for the crash has since come to light. One of those supposed causes was high-frequency trading, according to a report from the Securities and Exchange Commission that year.