Hard Fork Risks and Why 95% Should be the Standard.
There has never been a hard fork of Bitcoin, as most would define it, where the entire network needs to coordinate a protocol update at the same time. The closest Bitcoin came was in 2013, due to an unexpected software error and it caused significant disruption, and in 2012 where the protocol made a backwards incompatible change but it only affected clients over 2 years old which as far as developers could tell were not in use.
http://bitledger.info/hard-fork-risks-a ... ign=buffer