Ethereum Classic (ETC) suffered a 3,693-block chain reorganization early Saturday morning, an event first thought to be a possible 51% attack, after a miner used old software after having been offline, according to Terry Culver, CEO of Ethereum Classic Labs.

Update (August 1, 15:17 UTC): This article has been updated with an estimate of the attack cost.
Update (August 1, 16:28 UTC): This article has been further updated to reflect that the event was likely not a malicious act, but an accidental one in which an offline miner came back online using an old version of ETC. That event then mimicked a 51% attack.

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