Can unupgraded nodes that accept zero confirmation be fooled after SegWit? Considering the following scenario:


  After deployment of SegWit, there remain two old clients (with software that is unaware of SegWit) A &amp; B.
  A funds a transaction to pay B using a SegWit output which appears to both as ANYONE_CAN_SPEND.
  Upgraded nodes ignore this transaction, as it obviously isn't signed correctly under the rules of SegWit. (Perhaps even ban old nodes that relay this "invalid transaction" to them?) However, among old nodes the transaction gets propagated as valid. 


In the case that B gets notice of the transaction, would it display an "incoming unconfirmed transaction"?
Could people get fooled that way if they accept Zero Confirmation (which I'm aware isn't safe)?
If it is a transaction that pays B, that means that B chose that address. If B doesn't support SegWit, it won't ask for a SegWit output, so this cannot happen.

In practice, SegWit wallets will likely use P2SH initially; the SegWit output will be embedded in a pay-to-scripthash, and the address a SegWit wallets gives to sender will be the P2SH hash of the SegWit output script. However, in our scenario, B doesn't know anything about all that, and will just give out a normal old-style pay-to-pubkey-hash address, and A will pay to that.

More information about failure scenarios around softforks that introduce new transaction types: https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2015-December/012014.html