If I had to, I'd guess that AWS's messaging/monitoring/control infrastructure is likely to be the SPOF, as in the 2008 outage: http://status.aws.amazon.com/s3-20080720.html It's an obvious weak spot in the independence/isolation of AWS' nodes, and it would seem to be the one most likely to cause failure to reach across more than one AZ. (Apart from a stampede from one, affected AZ to others perhaps.)