The underlying layers of networking failing-

1. Too many requests for an NIC to handle,

2. Certificates expiring,

3. Firewall changes closing an active port down,

4. A router failing,

5. Congestion, interference in wireless networks resulting in excessive packet loss.

Hardware failing- HDDs, RAM, routers crashing, cables at datacenter being accidentally unplugged, racks being misconfigured during upgrades, maintenance etc.

See slide #4 here- http://static.googleusercontent.com/media/research.google.co...