Value overflow incident

On August 15, 2010, a transaction exploiting an integer-overflow bug (later CVE-2010-5139) was mined at block height 74,638, creating outputs totaling roughly 184.47 billion BTC — two outputs of ~92.2 billion each. The validation code summed outputs into a signed 64-bit integer that wrapped around, passing checks. Developers (the anomaly was flagged publicly by Jeff Garzik) shipped a fixed client within about five hours; nodes on the corrected rules rejected the bad block, and the repaired chain permanently overtook the exploited one within about a day (around block 74,691). No inflation persisted.

Nuance: This remains the most serious consensus bug ever exploited on Bitcoin mainnet, and the honest version of the story cuts both ways: the 21M schedule was violated for several hours, and the network's rapid social-technical response removed the violation. A related inflation bug in 2018 (CVE-2018-17144) was found and patched before any exploitation.

Common misstatements:

• "Bitcoin's supply cap has never been broken." — It was, for hours, in August 2010; the exploited chain was abandoned and the cap restored.
• "The overflow coins are still out there." — The block containing them is not part of the Bitcoin chain; nothing from that transaction exists in the UTXO set.
• Calling the fix "a rollback by developers." — Nodes adopting corrected rules rejected the invalid block; the chain reorganized under normal most-work rules among valid blocks.