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Bitcoin Data GuideBitcoin Reference

Network Stories · Protocol Incidents

The March 2013 Chain Split

On 11 March 2013 the chain split in two. A database change in version 0.8 made nodes disagree about which of two valid blocks to build on, and it took six hours and a deliberate rollback for the network to converge again.
Written by
The Bitcoin Data Guide editorial team
Reviewed by
Protocol review desk
Dates
Published 2026-09-21 · Last reviewed 2026-09-21

Fact

Two chains existed simultaneously for about six hours, and the network converged on the longer one.

Forecast

Which chain was 'correct' is a question about the rule, not the record; the network followed the longer chain.

What the record shows

A block is valid if it satisfies the consensus rules, and more than one valid block can exist at the same height. When that happens, nodes follow the chain with the most accumulated work, and the shorter branch is abandoned. The rule is designed to resolve exactly this situation without any coordination.

In March 2013 a block was mined that version 0.8 accepted but older versions rejected, because the two versions used different databases to look up the block index and disagreed about a limit on the number of signature operations. Nodes running 0.8 built on the new block; nodes running older versions built on a different one. For about six hours there were two chains.

The resolution was to restore consensus rather than to argue about which chain was right. A temporary rule was added to version 0.8.1 that made it reject the block the older nodes rejected, and miners moved to the longer branch. The network converged, and the shorter branch was abandoned.

Timeline

  1. 11 March 2013A block is mined that version 0.8 accepts and older versions reject, splitting the chain.
  2. 11–12 March 2013Two chains exist for about six hours while the cause is identified.
  3. 12 March 2013Version 0.8.1 restores consensus, miners move to the longer chain, and the split ends.

What the record does not settle

The split is sometimes described as a failure of consensus. The consensus rule worked as designed: the network followed the longer chain. What failed was the agreement between two software versions about which blocks were valid, and that is a software problem rather than a rule problem.

The soft forks and hard forks page explains how rule changes are made, and the node verification page covers the checks that decide which chain a node follows.

Sources

Bitcoin Core release notes — 0.8
The release that changed the block-index database and introduced the divergence.
Bitcoin Core release notes — 0.8.1
The release that added a temporary rule to restore consensus with older nodes.