Despite the ingenuity of this system, double spending of Bitcoin has always been theoretically possible. A bad actor or group of bad actors could take control of over half of the hash power on the network and use that to permit it to make transactions then overwrite those blocks with new ones that return payments to their own wallet. This is a so-called 51% attack.

But this was always considered very unlikely. It would be hugely expensive, in terms of both hardware and electricity to secure over half the network. In practice, there may be a limit to the share of computing power that can be taken over, especially in networks already dominated by a small number of large pools.

Even if such a takeover were possible, it was thought that the public nature of the ledger would ensure that other users would be alerted to the double spend very quickly. Once aware of the attack, a token could be forked or simply sold by investors. As such, it was supposed that any attack would undermine confidence in the currency, and its price, to such a degree, that it would be entirely self-defeating.

In other words, there was thought to be no incentive for anyone to seek to falsify the ledger, as doing so would crash the market and negate any potential upside.