If you create static DNS entries, then disable them, then flush the DNS cache, the static entries remain cached until the next reboot. There does not seem to be a way to flush them besides rebooting.
I understand this isn't quite a big problem for 99.99% of people, but it's a major pain in the ass for me in my home network, when testing servers (i.e. swapping back and forth between "true" DNS entries and temporarily swapped IP addresses for the test servers).
This is on 6.2, RB2011, by the way.