51% Attack

A 51% attack occurs when a single entity or group gains control of more than 50% of a blockchain's hashing power or computational capacity. This level of control enables the attackers to perform several malicious activities such as double-spending (reversing transactions after they have been confirmed), blocking other participants' transactions, and monopolizing mining rewards. The attack undermines the decentralized nature of the blockchain and can significantly harm its reputation and security. 

In 2019, the Ethereum Classic (ETC) blockchain suffered a 51% attack where the attackers reorganized the chain and double-spent approximately $1.1 million worth of ETC. This event shook investor confidence in smaller blockchains with lower hashing power, as they were more vulnerable to such attacks.