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Segiwt2x, software behind an agreement of 80% of the mining hashrate and some major bitcoin companies, launched a separate testnet several weeks ago in order to provide an environment in which this new piece of software can be developed and see what potential effects it would have on the real Bitcoin network. The testnet is a completely separate blockchain, with separate miners, separate coins, separate nodes, and most importantly, separate consensus rules that decide whether or not a block is valid.
btc1 testnet irrevocably forks. https://t.co/GNH84Jblq4
— UASF Cosmic (@bhec39) 10 июля 2017 г.
The accidental fork thing occurred on Monday, July 10. It caused the network to essentially go down for almost a full day along with creating two incompatible testnets. Apparently, not a single block was created during this period as developers rushed to find the cause and to work out a solution to the problem.
Distributed database that is used to maintain a continuously growing list of records, called blocks
As it turned out, the reason for the fork was that when the chain hit a certain block number, the first block that began the chain allowing for 2MB blocks needed to be over 1MB. Since it is a testnet and lacks a real user base, the mempool was essentially empty. There weren’t enough transactions being made to add up to greater than 1MB, causing the fork.
Although unexpected timing, this is otherwise a good field test.
The leader of the Segiwt2x team
In a message on the Segwit2x working group mailing list, the leader of the team Jeff Garzik reported that the fork is a result of a testnet miner activating the fork without the preparation needed from the rest of the people on the testnet. While it is obviously a problem that needs to be fixed, developers argue that the point of the testnet is to test new software.
Experts are also concerned about the problem, assuming that the fork could affect the outcome of the vote. If the number of miners decreases, the implementation of Segwit2x might be postponed.