Some miners had signaled readiness for the soft fork while continuing to mine on headers from other pools without validating the underlying blocks. A six-block invalid branch and a later three-block branch were overtaken by the valid chain.
All events
BIP 66 activates and exposes false signaling
Bitcoin began enforcing strict DER signatures, after which miners building without full validation produced short invalid chains.
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IN THE SHOWSThe Fork WarsThe Machine