After an investigation by our sysadmin team, it appears that the issue was the result of a faulty kernal patch. The patch required a manual reboot of the VM, which resulted in approx. 7 minutes of downtime. Previous to this 7 minutes of downtime, this cluster has been up 100% for 686 days.
The montreal1000 cluster is now responding & serving sites. It was down for 6 minutes and 6 seconds. Details will be posted here on the cause once our sysadmins properly investigate.
We're currently investigating an outage that is affecting the montreal1000 cluster.