@blakeh - I've seen a bunch of messages like that too on Reddit an Twitter...

Absolutely brutal. (I always trade with limit orders and never use margin so my only regret is not having set a limit order that seemed crazy at the time.)

There is metion of an ICO that caused it on ZeroHedge. http://www.zerohedge.com/news/2017-06-21/ethereum-flash-crashes-after-status-ico-clogs-network