Around an hour ago, the Stack Overflow and other Stack Exchange sites were in read-only mode, which as far as I have seen wouldn't let me log in, so I was unable to post any questions, answers, or comments. However somehow I saw some new questions suddenly appearing (I noticed this on both Stack Overflow and Meta Stack Overflow). How is that possible?
The Stack Exchange read-only test is starting now, DNS failover will happen over the next 5 minutes.
That would lead me to believe that they tinker with DNS records to make the failover to the secondary read only data center in Oregon happen. Stack Exchange has set the time to live on their DNS records to 300 seconds. Meaning the information that translates stackexchange.com into an IP address should only be cached for 300 seconds before asking Stack Exchange for the information again. If your computer or ISP decides it won't obey that 300 second cache limit, that user would have seen the New York version of Stack Exchange which was not in read only mode.
Obviously, Nick (or another Stack Exchange employee) may have a different account of the events. But I've had a similar situation happen in the past.