beaker-provision does not enforce any read timeout when fetching images for the configure_netboot command. That means if the HTTP server is misbehaving and never returns a response, the configure_netboot command could end up stuck in limbo until beaker-provision is restarted.
On Gerrit: http://gerrit.beaker-project.org/3171
Verifying this outside of devel is quite difficult. You would need to edit a distro tree's URLs to point at a custom HTTP server which accepts requests but then waits forever without sending a response or closing the connection. The expected result is that beaker-provision eventually reports that the command failed with "urlopen timed out". There is a reproducer in the test suite, using a fake HTTP server. I don't think it's worth trying to verify the fix by hand in a real environment.
Beaker 0.17.1 has been released.