The changes made to the chain by the hard-fork was a one-time balance modification to several accounts. As of block 1920000, the difference between the state tries of the two forks was in the balance of a half-dozen accounts.

Have other blockhains (other than Ethereum) done similar state-modifying forks, or is Ethereum the first to test a similar balance-modification fork ? What are some examples one could compare the DAO-rescue fork with ?

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Early in Bitcoin's history, there was a rollback to the most severe bug the platform had ever faced, arguably still today.

8th August 2010 - 92 billion BTC came into existence

On 8th August 2010 bitcoin developer Jeff Garzik wrote what could be mildly described as the biggest understatement since Apollo 13 told Houston: “We’ve had a problem here.”. “The ‘value out’ in this block is quite strange,” he wrote on bitcointalk.org, referring to a block that had somehow contained 92 billion BTC, which is precisely 91,979,000,000 more bitcoin than is ever supposed to exist. CVE-2010-5139 (CVE meaning ‘common vulnerability and exposures’) was frighteningly simple and exploited to the point of farce by an unknown attacker. In technical language, the bug is known as a number overflow error.So instead of the system counting up 98, 99, 100, 101, for example, it broke at 99 and went to zero (or -100) instead of 100. In layman’s terms, someone found a way to flood the code and create a ridiculously large amount of bitcoin in the process.

The fix was the bitcoin equivalent of dying in a video game and restarting from the last save point. The community simply hit ‘undo’, jumping back to the point in the blockchain before the hack occurred and starting anew from there; all of the transactions made after the bug was exploited – but before the fix was implemented – were effectively cancelled.

How serious was it? Bitcoin’s lead developer Wladimir Van Der Laan is pretty blunt about it, telling me: “It was the worst problem ever.”

The difference is that this Bitcoin fork was to fix a protocol level bug while Ethereum's fork was to fix a bug of a smart contract within the platform.

Sources: https://bitcointalk.org/index.php?topic=702755.0 ; http://www.coindesk.com/9-biggest-screwups-bitcoin-history/ ;

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    The other difference is that after this rollback the protocol and algorithms to verify the blockchain remained unchanged, as it was an implementation error. Whereas in the Ethereum hardfork, a state change was introduced that needs to be added to the specification so that clients can verify the integrity of the blockchain. Aug 10, 2016 at 12:34

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