On my MythTV box running Arch Linux, I boot from an SSD. This means that stuff happens *really* fast at startup - sometimes *too* fast.
The situation is that my eth0 device is not always initialized before the network.service service starts. This results in the service failing and no network:
Jan 02 20:44:29 mingus.lan systemd[1]: Starting Static IP...
Jan 02 20:44:29 mingus.lan ip[385]: Cannot find device "eth0"
A quick google revealed the solution on the Arch Linux forums. The long and short of it is add a dependency for your NIC on network.service. Get the device name like so:
[jnthornh@mingus system]$ systemctl --full | grep net-eth0
sys-devices-pci0000:00-0000:00:0a.0-net-eth0.device
Toss it in the "Unit" section of /etc/systemd/system/network.service:
Requires=sys-devices-pci0000:00-0000:00:0a.0-net-eth0.device
After=sys-devices-pci0000:00-0000:00:0a.0-net-eth0.device
And you're good to go.