On Dec 18, 2015, at 10:30 AM, Pieter Wuille via bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org> wrote:

> 1) The risk of an old full node wallet accepting a transaction that is
> invalid to the new rules.
> 
> The receiver wallet chooses what address/script to accept coins on.
> They'll upgrade to the new softfork rules before creating an address
> that depends on the softfork's features.
> 
> So, not a problem.


Mallory wants to defraud Bob with a 1 BTC payment for some beer. Bob runs the old rules. Bob creates a p2pkh address for Mallory to use. Mallory takes 1 BTC, and creates an invalid SegWit transaction that Bob cannot properly validate and that pays into one of Mallory's wallets. Mallory then immediately spends the unconfirmed transaction into Bob's address. Bob sees what appears to be a valid transaction chain which is not actually valid.

Clueless Carol is one of the 4.9% of miners who forgot to upgrade her mining node. Carol sees that Mallory included an enormous fee in his transactions, so Carol makes sure to include both transactions in her block.

Mallory gets free beer.

Anything I'm missing?