No IPv4 at Philadelphia IETF
At the IETF-71 in Philadelphia it was a planned outage of IPv4. There was also some outage of electricity before the outage of IPv4. Some people might remember this as The Philadelphia Experiment.
I think these experiments are good, as one would otherwise never actually fight with IPv6 configuration on the routers, software etc. I have spent some hours on this. A good thing.