Yesterday I saved a machine state while the host had a USB Networking interface connected and being used as a bridged adapter.
Restarting that Saved Machine State today, I got
Failed to open a session for the virtual machine
Nonexistent host networking interface
Unknown error creating VM
As the settings for a saved machine are unavailable, there are two solutions, re-connect the USB Networking interface, or hack on the Machine XML file.
However, I would suggest that there is no reason for this to be a fatal error. The VM could continue by simply disabling the Network Interface, or using the next available host network interface for the bridge.