"An attacker that controls more than 50% of the network's computing power can, for the time that he is in control, exclude and modify the ordering of transactio

"An attacker that controls more than 50% of the network's computing power can, for the time that he is in control, exclude and modify the ordering of transactions. This allows him to: * Reverse transactions that he sends while he's in control Prevent some or all transactions from gaining any confirmations * Prevent some or all other generators from getting any generations" Source: https://en.bitcoin.it/wiki/Weaknesses#Attacking_all_users https://en.bitcoin.it/wiki/Weaknesses#Attacking_all_users