A 51% attack occurs when someone controls the majority of the mining power on a blockchain proof-of-working network. That means the majority block checker can prevent the mining and reverse transactions by other users. While many assumed that a 51% attack would be perpetrated with malicious intent, the above-mentioned case occurred when the two mining pools sought to prevent an unaidentified party from taking coins which, because of the Code update, were mainly “up for grabs.” Swann said that the attack was perpetrated in an effort by two miners with the majority control of the network — BTC.top and BTC.com  According to Swann’s tweets: 51% attacks were generally considered to be unwanted and unprofitable, because they require huge amounts of computing power and once the network was considered compromised, users would flee ostensibly. Thread 👇🏻 1/ What I’ve gathered from loose details: First, there was an unintentional split with the recent #BCH “upgrade.” — Guy Swann⚡ (@TheCryptoconomy) 24 May 2019 BTC.top and BTC.com control 43 percent of the Bitcoin cash mining pool according to Coin. Dance statistics. As Cointelegraph reported, the blockchain of the Ethereum Classic (ETC) was attacked 51% in January. Crypto exchange researchers Gate.io reported that an assailant had overturned four transactions and lost 54,200 ETC. The exchange promised to compensate the users affected and recommended the blocking of transactions started by the attacker to other trading platforms.