On Sat, Feb 6, 2016 at 3:46 PM, Luke Dashjr via bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org> wrote:

On Saturday, February 06, 2016 5:25:21 PM Tom Zander via bitcoin-dev wrote:
> On Saturday, February 06, 2016 06:09:21 PM Jorge Timón via bitcoin-dev
wrote:
> > None of the reasons you list say anything about the fact that "being
> > lost" (kicked out of the network) is a problem for those node's users.
>
> That's because its not.
>
> If you have a node that is "old" your node will stop getting new blocks.
> The node will essentially just say "x-hours behind" with "x" getting larger
> every hour. Funds don't get confirmed. etc.

Until someone decides to attack you. Then you'll get 6, 10, maybe more blocks
confirming a large 10000 BTC payment. If you're just a normal end user (or
perhaps an automated system), you'll figure that payment is good and
irreversibly hand over the title to the house.

There will be approximately zero percentage of hash power left on the weaker branch of the fork, based on past soft-fork adoption by miners (they upgrade VERY quickly from 75% to over 95%).

So it will take a week to get 6 confirmations.

If you are a full node, you are warned that your software is obsolete and you must upgrade.

If you are a lightweight node, it SHOULD tell you something is wrong, but even if it doesn't, given that people running lightweight nodes run them so they don't have to be connected to the network 24/7, it is very likely during that week you disconnect and reconnect to the network several times. And every time you do that you increase your chances that you will connect to full nodes on the majority branch of the chain, where you will be told about the double-spend.

All of that is assuming that there is no OTHER mitigation done. DNS seeds should avoid reporting nodes that look like they are in the middle of initial block download (that are at a block height significantly behind the rest of the network), for example.

--
Gavin Andresen