The blockchain technology is a partial solution to the Byzantine generals problem. A full solution to the problem has been mathematically shown to be impossible. As a result, blockchain systems are vulnerable to attack by introducing enough fraudulent voting power to improperly modify the blockchain.
Apparently this is now happening to some of the smaller networks.
The potential prizes on the larger ones are rich enough that it is probably just a matter of time until one of them is compromised for serious money. Then we’ll see what happens.
Financial Reality 