Not true. A malicious 50% miner can cancel any transaction it wants, not just fix doublespend. Assuming the transaction was sufficiently recent (the more hash power they have the more such a transaction can be in the past).<p>So they can &quot;unspend&quot; their own bitcoins, as follows : use 51% of your hash power to create a parallel blockchain, which is not published. Include all transactions, except the one you used to buy a TV. Put something else in it&#x27;s place. Keep doing this until the TV is shipped&#x2F;arrived. Then publish the parallel blockchain. Boom. Unspent.