Essentially, Ghash.io would be in control of the Bitcoin blockchain, allowing double spending of BTC and other nefarious deeds. See https://en.bitcoin.it/wiki/

Essentially, Ghash.io would be in control of the Bitcoin blockchain, allowing double spending of BTC and other nefarious deeds. See https://en.bitcoin.it/wiki/Weaknesses#Attacker_has_a_lot_of_computing_power https://en.bitcoin.it/wiki/Weaknesses#Attacker_has_a_lot_of_... Edit: According to the Wiki linked, these attacks are also possible with <50% mining rate, although with lower probability of success. So this is a problem even if Ghash.io does not crack the 51%.