Either the author of this article is confused or I am. He describes the situation of a deliberate hard fork into two chains with different consensus rules. He then notes that Alice can spend her original (prefork) bitcoin differently on the two resulting chains --- but where he loses me is where he claims that this somehow represents a double-spend vulnerability.
Yup. When Bitcoin forked into Bitcoin cash, everyone was free to "double spend" - once in Bitcoin and once in Bitcoin cash. Money just up and doubled itself.
An actual great example is in 2013 when this happened [0]. For 24 hours, Bitcoin could not reach consensus and users could double-spend.
The fact that we do not know how likely Bitcoin consensus failure is—especially when so many forks and conflicting rulesets are emerging—should worry anyone thinking about decentralized consensus.
“Of course” we have 2 ledgers and no consensus between them? So do we all agree that Bitcoin does not solve decentralized consensus and I missed the memo?
In the first step, he says roughly "assume half of the nodes change protocol". I named by dog Bitcoin too but he doesn't have the same UTXO state as the main chain.
If Alice changed the ruleset so drastically, I don’t think she could find any Bob that would be willing to accept this thing as Bitcoin. But maybe the probability is not 0.
If Alice only changed 1 little rule and made it conflicting with the ruleset used by the rest of the network, it’s easy to imagine consensus failure and being able to find 1 Bob that gets tricked into receiving a double-spent Bitcoin. In fact, such a system failure has happened already in 2013, when for 24 hours no one knew which ledger was the real bitcoin: https://bitcoinmagazine.com/articles/bitcoin-network-shaken-...
