the DAO hack
lorecryptography
The 2016 drain of the largest crowdfunded smart contract, answered by the hard fork that split Ethereum in two.
The DAO raised a record crowdfund as an investor-directed venture contract; in June 2016 an attacker used a reentrancy flaw to siphon about 3.6 million ether, then worth roughly 50 million dollars. Ethereum's July 20 hard fork rewound the theft, and dissenters who held that code is law kept the original chain alive as Ethereum Classic. It is smart contracts' founding disaster and the permanent case study in what immutability costs when the immutable thing is a bug.