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At around 16:00 EST, there was a brief but massive flash-crash visible on GDAX:

http://www.cnbc.com/2017/06/22/ethereum-price-crash-10-cents-gdax-exchange-after-multimillion-dollar-trade.html

People who had buy orders for Ether at low prices made spectacular gains in the ensuing rebound, while those who had sell orders for low prices had terrible losses.

Why/how did this occur?

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From https://blog.gdax.com/eth-usd-trading-update-5d8142b5bdc1:

On 21 June 2017 at 12:30pm PT, a multimillion dollar market sell was placed on the GDAX ETH-USD order book. This resulted in orders being filled from $317.81 to $224.48, translating into a book slippage of 29.4%. This slippage started a cascade of approximately 800 stop loss orders and margin funding liquidations, causing ETH to temporarily trade as low as $0.10.

Our initial investigations show no indication of wrongdoing or account takeovers. We understand this event can be frustrating for our customers. Our matching engine operated as intended throughout this event and trading with advanced features like margin always carries inherent risk.

Any orders/sells placed below 224.48 were the result of cascading stop losses and various algorithmic trading implementations. The original market sell was a simple matter of extreme liquidation. No different than if an individual had decided to liquidate their entire retirement account, including 401ks and IRAs.

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  • I wish I'd had a very low buy order in. – Pixel Jun 22 '17 at 22:34