The DAO hack

The DAO was a smart contract fund that raised about 12 million ETH, around 14 percent of all ether at the time, in May 2016. On June 17, 2016 an attacker used a reentrancy bug in its code to repeatedly withdraw funds before the balance updated, moving about 3.6 million ETH into a child contract.

The price of ether fell sharply in the days after the attack. Because of a built in waiting period, the attacker could not move the funds right away, which gave the community time to debate what to do.

After weeks of argument, the community chose a hard fork at block 1,920,000 on July 20, 2016 that moved the drained ether into a refund contract. A minority rejected the change on principle and kept the original chain going as Ethereum Classic (ETC). The SEC later used The DAO in a 2017 report saying such tokens can be securities.