March 2013 fork

On March 11–12, 2013, Bitcoin accidentally split into two chains for about six hours (roughly 24 blocks). A large block at height 225,430, produced by a v0.8.0 node, exceeded a previously unknown Berkeley DB lock limit in pre-0.8 nodes (v0.8 had switched to LevelDB), so older nodes rejected a block newer nodes accepted. Coordinating in public IRC channels, developers and major pools (BTC Guild, Slush) downgraded to 0.7-compatible behavior so the chain old nodes could follow would win; the split resolved by block 225,454. BIP-50 is the post-mortem.

Nuance: The episode is cited in debates for two different lessons: that consensus can break via implementation divergence (not just rule disagreements), and that resolving it required rapid informal coordination among a small group — evidence used both for and against claims about Bitcoin's decentralization in practice. One double-spend against a payment processor (~$10k) was reported during the confusion.

Common misstatements:

• "Bitcoin has never had an accidental chain split." — March 2013 was exactly that.
• "The 2013 fork was caused by a protocol rule change." — It was an undocumented database-level limit; both sides believed they were following the same rules.
• Conflating this with the 2010 overflow incident or the 2017 BCH split — three distinct events with different causes (bug exploit, implementation divergence, deliberate hard fork).