It's theoretically possible for transactions per block to become so high, that in order to get in a block at all you have to put in a transaction fee, right? Well what happens, in that environment, if I put a transaction fee of 0, and my transaction is never ever picked up? How would I correct that situation?
If I increased the transaction fee amount, and tried again, and that one went through, would an attacker then be able to create his own block, with the first payment inside? Or is there a way to revoke transactions permanently before they end up in a block? Is there a 'deadline' on a signed transaction? i.e. 'after block X, this transaction would not be valid'?