For 7 hours and 41 minutes in March 2013 the Bitcoin network was split in two

On 11 March 2013 an upgrade split the network in two. New nodes followed one chain, old nodes could only follow another, and about 60 percent of the mining power was on the side that old nodes rejected. It got fixed that night, in public, in the developers' chat room. We went back through the log and the block data and wrote it up.

A few things that surprised us while doing it:

The block that caused it wasn't too big. It was 998 kB, under the limit. Old versions had capped their database at 10,000 locks years earlier, nobody thought of that as a rule about blocks, and it turned out to be one. It was even a slightly different rule on each machine.

The developers didn't agree at first. The lead maintainer's opening position was "first rule of bitcoin: majority hashpower wins." Pieter Wuille's answer was that a majority isn't enough if it leaves every merchant on old software behind. That argument took about twenty minutes.

The person who settled it was a pool operator. He ran about a third of the network, said "I can single handedly put 0.7 back to the majority hash power," asked the developers to confirm, and then moved his miners onto the shorter chain. It cost him 150 to 200 BTC.

Most accounts say 24 blocks and six hours. The nodes that reorganized logged "Disconnect 25 blocks," and from the triggering block to the reorg was 7 hours 41 minutes.

There was one double spend, about $10,000 against a payment processor. Both sides of it are still visible on a block explorer, and the money was returned the next day.

The chapter also covers the fix, which was only finished by a planned rule change that summer, and what the whole thing says about who decides the rules. It doesn't pretend the ending was tidy: it was fixable in a night because a handful of people could fix it, and that cuts both ways.

Every quote has a timestamp and every number has a source at the bottom of the page. If you were around for it and we got something wrong, say so and we'll fix it the same day and credit you.

https://www.learnbitcoin.com/rabbit-hole/2013-chain-fork