In May 2018 Bitcoin Gold (and two other cryptocurrencies) were hit by a successful 51% hashing attack by an unknown actor, in which exchanges lost estimated $18m. In June 2018, Korean exchange Coinrail was hacked, losing US$37 million worth of altcoin. Fear surrounding the hack was blamed for a $42 billion cryptocurrency market selloff. On 9 July 2018 the exchange Bancor had $23.5 million in cryptocurrency stolen.
These are NOT linked to or related in any way, to the futures contracts that trade on the CME or CBOE. Not every article I have read makes this clear. So the bright side of this story is that the contracts listed and traded on U.S. exchanges were not involved. That is encouraging from both a regulatory aspect and for the future potential growth of cryptocurrency linked products in the U.S.