Ethereum flash crashed on June 21st, 2017 on GDAX exchange.
Here's how it went.
This is what's called a Black Swan Event, and the reason why trading on margin is dangerous if you're not doing proper risk management. Slippage can not only erase your funds but put you in massive debt. Last one I remember seeing was on EURCHF when the Swiss National Bank decided to remove the 1.2 EURCHF peg. Here's the story on how people who were trading less than 10k ended up with debts over £250k. http://www.telegraph.co.uk/finance/personalfinance/investing/11562202/How-370-investors-lost-18m-in-minutes.html
Saxo Bank, a reputable Danish broker also had a rough time recording losses totalling $107M. They have since bounced back well and remain a trustworthy broker to do business with.
Be wary of margin, if you decide to trade on margin you have to make sure you are with a broker/exchange which allows you to hedge and with a fast connection and access to good liquidity.