Last November a hacker going by the name “DevOps199” found and exploited a flaw in the code for a subset of Ethereum wallets. That user was able to exploit teh vulnerability and take ownership of an Ethereum code library known as a smart contact. Once the hacker owned that smart contract, it was destroyed leaving about $150 million of Ethereum

Source: Hot Hardware – Researchers Uncover Ethereum Contract Vulnerabilities Putting Millions Of Dollars At Risk