On Aug 17, 2016 00:23, "Russell O'Connor via bitcoin-dev" <bitcoin-dev@lists.linuxfoundation.org> wrote:

> If one's goal is to mess with an transaction to prevent it from being mined, it is more effective to just not relay the transaction rather than to mess with the witness.  Given two transactions with the same txid and different witness data, miners and good nodes ought to mine/relay the version with the lower cost (smaller?) witness data.

That implies that everyone will see both versions and be able to make that choice. Unfortunately, those two versions will be definition be in conflict with each other, and thus only one will end up paying a fee. We're can't relay two transactions for the price of one, or we'd expose the p2p network to a very cheap DDoS attack: just send increasingly small versions of the same transaction.

Segwit's third party mallebility protection makes it not an issue for dependent contracts if transactions are mauled (=apparently the verb related to malleability), but there are still good reasons for senders not to gratuitously make their transactions extensible in size or other resources.

--
Pieter