Flash Crash: Quick drop-and-recover in securities prices that occurred shortly after 2:30 p.m. EST on May 6, 2010. Initial reports indicating that the crash was caused by a mistyped order proved to be erroneous. The causes of the flash crash remain unknown. Both the Securities and Exchange Commission and the Commodity Futures Trading Commission have investigated the incident and released a report that provided several working hypotheses, but failed to identify a single cause for the incident.

Source: Investopedia