The DAO

The DAO was an early Ethereum-based Decentralized Autonomous Organization (DAO) and crowdfunding project that once held 15% of all circulating Ether. After it was exploited for $60M, Ethereum was hard forked, resulting in Ethereum and Ethereum Classic, the latter of which rejected a decision to take the ETH back from the attacker. Originally shipped as an investor-directed venture capital firm, the DAO was one of the earliest crowdfunding projects built on Ethereum.

Vitalik Buterin, an Ethereum co-founder, proposed three primary solutions to the hack, including a soft fork that blacklisted the attacker, a hard fork that would rebuild the chain except for the attack and code vulnerabilities, or to do nothing at all. Throughout this process, the alleged attacker had insisted that he would resist any level of forking through validator bribes and legal action - as he claimed that he hadn't broken any laws to acquire the funds.

Ethereum eventually decided to follow through with the hard fork, creating the current version of Ethereum and Ethereum Classic.