Just wanted to post a find, recently we were having issues with our NetBoot going haywire on a 10.8 server (migrated from 10.7 and it didn't happen on 10.7 server) every so often, and the culprit was that the HD space for the NetBoot session was being taken up. I've figured out that it was sleepimage being created in /private/var/vm which takes up as much space as physical RAM is installed.
So following https://jamfnation.jamfsoftware.com/article.html?id=198 the minimum space for your Netboot sets should be at least the size of the Netboot + largest amount of RAM you have in your environment, or better, create a Netboot set that make sure you disable sleepimage ever being created.
pmset -a hibernatemode 0 sudo rm /private/var/vm/sleepimage touch /private/var/vm/sleepimage chflags uchg /private/var/vm/sleepimage
