A minority group of miners can obtain revenues in excess of their fair share, and grow in number until they reach a majority. When this point is reached, the Bitcoin value-proposition collapses. This snowball scenario does not require an ill-intentioned Bond-style villain to launch; it can take place as the collaborative result of people trying to earn a bit more money for their mining efforts. We outline a practical fix to the Bitcoin protocol that is easy to deploy and will guard against the attack as long as 3/4ths of the miners are honest.”]
Source: https://hackingdistributed.com/2013/11/04/bitcoin-is-broken/