Yesterday, the Bitcoin network experienced one of the most serious hiccups that we have seen in the past four years. Starting from block 225430, the blockchain literally split into two, with one half of the network adding blocks to one version of the chain, and the other half adding to the other. For the next six hours, there were effectively two Bitcoin networks operating at the same time, each with its own version of the transaction history. The split lasted for 24 blocks or 6 hours, finally resolving itself when one version of the chain conclusively pulled ahead of the other at block 225454, leaving the other chain largely abandoned, with only a small number of miners that are incapable of recognizing what has now become the main chain still mining it, while the bulk of the network quickly returned to normal.

The fork was first noticed at about 23:30 GMT on Monday, March 11, when “thermoman” on the bitcoin-dev IRC channel mentioned that “some client told my client it (the other host) had 225431 blocks, but blockexplorer says that currently the block count is at 225430″. Some other blockchain resources also showed 225430 blocks. Over the course of the next thirty minutes, other users started reporting more strange reports from Bitcoiin client logs. Bitcoin developer Peter Wuille (“sipa” on IRC) claimed that he was on block 225435, and then soon 225439, while other sources were still reporting 225431. At 00:00 GMT March 12, sipa posted “I wonder if there’s something that triggered it on the network, a large reorganization or so”. It turned out that a blockchain reorganization, an event that happens when a client discovers a new blockchain longer (and therefore more likely to be valid) than the one it was working with before, and switches to it, was indeed what happened, and over the next few minutes everyone realized what had happened: a blockchain fork.