Or "Don't reboot your net-booting clients if you've just upgraded your DHCP server"
For reasons of RFC correctness, the ISC has changed the behaviour of their DHCP server, within a maintenance release, between versions 3.0.2 and 3.0.3.
Specifically, the next-server attribute defaults to zeros if not set, whereas previously it defaulted to the address of the DHCP server. So, upgrading has the potential to break setups that previously worked.
It doesn't help that the upstream manpage for dhcpd.conf doesn't appear to have been updated to reflect this change. That'd be #327829.
I put a NEWS.Debian into the 3.0.3 release of the package that's just hit testing, but I'm wondering if in the interests of full(est) disclosure, it's better to use a selectively displayed debconf note instead? I think I'll do that for the next revision of the package, just to avoid surprise, as the NEWS.Debian file doesn't get translated, and doesn't really get put in your face unless you have apt-listchanges installed and appropriately configured.