1) Someone appears to be modifying "a large portion of all transactions." 2) You don't have to make it to a majority, you only have to propagate your transac

1) Someone appears to be modifying "a large portion of all transactions." 2) You don't have to make it to a majority, you only have to propagate your transaction to the miner who wins the next block prior to the original broadcaster propagating their transaction to miner of the next block. This is really easy to arrange in a P2P network by, e.g., controlling more nodes than the original party or by e.g. modifying the standard Bitcoin client to not sleep between attempts to propagate transactions or by e.g. communicating preferentially with nodes known to be controlled by or "close to" miners and not bothering to propagate to the 99.X% of nodes which are not. If you control thousands of nodes, which you can trivially do ("rent a botnet"), you will ROFLstomp the propagation capabilities of most people originating transactions. The ease of creating extra nodes is why Satoshi designed Bitcoin as "One CPU one vote" (for a transaction's correctness) rather than "One IP one vote." Unfortunately, it appears that Satoshi did not anticipate that the transaction hash is, effectively, the latter. That wouldn't be problematic except systems which interface with Bitcoin frequently rely on the transaction hash being stable but it is, in effect, written on water for the first hour or so.