UPDATE (22, November 12:30 UTC): The rogue bitcoin cash chain ceased mining on the old consensus protocol, Bitcoin ABC 0.19.0, after 25 blocks, according to BitMEX Research.

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The on-going chain battle, which should be resolved by bitcoin cash's internal rules, gives an insider look into how proof-of-work (PoW) consensus mechanisms operate.

The hard fork occurred on Friday at 16:49:28 UTC at block number 609,135. Two additional blocks broke into two different chains that have been mined for a total of 4 new blocks, according to BitMEX Research’s Fork Monitor.

The Bitcoin Cash chain split continues....

The shorter original rules chain has been extended by one addiitonal blockhttps://t.co/U9hbK4peip pic.twitter.com/lynovM9oVb

— BitMEX Research (@BitMEXResearch) November 15, 2019

The old chain, Bitcoin ABC 0.19.0 mined by the unknown pool, considers the new chain invalid while the new chain, Bitcoin ABC 0.20.06 mined by mining pool BTC.com, considers the old chain invalid.

But under the Nakamoto consensus mechanism employed by bitcoin cash and other proof-of-work (PoW) protocols, the chain with the longest history would be considered to be the trusted ledger, invalidating the rogue chain.

Seeing old chains continue has happened before as some miners, who run software that creates the blocks forming the chain, forget to upgrade to the new software required for hard forks before the forking date.

As BitMEX Research noted in a tweet, mining the wrong chain comes at a cost.

The opportunity cost for mining the incorrect chain could reach up to 25 BCH, or about $6,600, if the unknown pool had beat other miners in securing the two correct blocks, in addition to the electricity costs associated with mining.

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