The BIP66 update was meant to align miners on stricter cryptographic signatures through a form of block chain Democracy. As part of the BIP rules, once 950 of the past 1,000 blocks upgraded to the newer version all miners would older version blocks. This morning the network met that goal and a small, non-upgraded miner mined an older version block. Instead of rejecting the block roughly half the network accepted it and the Bitcoin block chain forked between software versions.
The bogus chain began when the BTCNuggets mining pool mined an out of date block. Though the main chain orphaned the block lightweight and older version of Bitcoin Core wallets are still at risk. According to Bitcoin.org’s network alert nearly 50% of the network hash rate mines on these lightweight or old versions. Any user running the older versions should stop accepting transactions immediately and upgrade their software.
“Early morning UTC on 4 July 2015, the 950/1000 (95%) threshold was reached. Shortly thereafter, a small miner (part of the non-upgraded 5%) mined