This can sometimes happen after you have sysprepped an machine and missed the boot menu prompt, entering the Out-of-box-experience. Trying to shutdown the device will leave the dirty bit resulting in the unclean error.

To avoid having to sysprep again, allow the device to enter OOBE again and hit shift+F10 and enter “oobe\bypassnro” at the resulting command prompt. Once you hit enter, the system will perform a clean reboot, clearing the dirty bit and allowing Fog to capture the filesystem correctly.

Hopes this helps someone else as some Dell machines are notorious for failing to get an IP from IPV4 PXE boot.