ETH/USD dropped from around $300 to $13 in a matter of seconds. Whatever happened, GDAX is in trouble right now. Whoever had a buy-order open is probably lucky. Like the US$ 101 BTC guy on btc-e a while ago.
GDAX is currently hardly accessible.
Crypto Traders Room is actually broadcasting live about the event on YouTube:
Most likely scenario: Someone with a lot of ETH to sell clicked accidentially on Market Sell instead of Limit Sell. Hopefully this guy got his Ether cheap... and
Crypto Traders Room got his account liquidated. He had 112 ETH now 6. Worst case scenario when doing margin trading.
Update: GDAX published an explanation:
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.
You can read the full blog post here:
https://blog.gdax.com/eth-usd-trading-update-5d8142b5bdc1
For the person causing the crash it was presumably not such a big loss. Assuming it was an accident he or she sold at a 30% loss max. only. However, for everyone having a margin trade open the consequences result in a near total loss. And others had a buy-order filled at just 10 cents. Insane profit.
It is suspected by some people, that the ETH sold was originally sent from this wallet:
https://etherscan.io/address/0x7d551397f79a2988b064afd0efebee802c7721bc
Thinking about it... when there is many margin trades open and someone sells a huge amount of ETH at market price, the same person could have placed huge buy orders at insanely low prices. By catching the down-cascade of executed liquidations this person could make a profit beyond imagination. Basically "stealing" the money from every margin trader on the broaker.
(silly typo, now this post is in etc, sorry, added the eth tag)