In 2016, a notorious attack on a decentralized autonomous organization (DAO) called The DAO exploited vulnerabilities in the Solidity code, illustrating a critical flaw known as a reentrancy attack. This attack occurred during the nascent stages of Ethereum, a blockchain platform that supports smart contracts, which are self-executing contracts with the terms of the agreement written into code. The DAO, a decentralized investment fund, had amassed $150 million worth of ether through token sales. However, within three months, a hacker used a reentrancy attack to steal this ether by exploiting a fallback function in the smart contract, allowing recursive withdrawals before balance updates. This incident sparked debate over the immutability of the blockchain, ultimately leading to a controversial decision to hard fork Ethereum, creating two separate blockchains: Ethereum Classic and the current Ethereum network. The hack demonstrated the need for secure coding practices, prompting solutions like balance updates before transfers and the use of reentrancy guards to prevent such vulnerabilities.