There have been two. The incident you're thinking of was accidental at first, then done deliberately with notice two months later: http://siliconangle.com/blog/

There have been two. The incident you're thinking of was accidental at first, then done deliberately with notice two months later: http://siliconangle.com/blog/2013/05/13/bitcoin-blockchain-hard-fork-coming-may-15th-final-warning/ http://siliconangle.com/blog/2013/05/13/bitcoin-blockchain-h... When I wrote my comment I was also thinking of https://bitcointalk.org/index.php?topic=827.0 https://bitcointalk.org/index.php?topic=827.0 from 2010, but on a cursory glance it looks like that was resolved with a soft fork, which is possible because the rules were made more restrictive rather than more permissive.