This happened in January 2015 when SNB (Swiss National Bank) removed the EURO and Swiss Franc peg.
No one was expecting this announcement as Swiss National Bank had just announced a few weeks back it will keep the peg.
These type of flash crashes are becoming more and more common now a day.
The recent example is that of the British Pound Flash Crash.
No one knows what happened.
GBPUSD fell more than 1400 pips in 2 minutes.