A prerelease does not only build your selected repositories but also downstream packages which depend on your packages (see "What gets tested" under http://wiki.ros.org/regression_tests). In the log you will see that it first builds your packages.
Afterwards it determines which other packages depend on yours:
Get list of wet repositories that build-depend on repo list: nasa_r2_common
Build depends_on list of pkg list: rosjava_extras, gazebo_interface, rosjava_core, rosjava_messages, r2_controllers_gazebo, r2_gazebo
Then those repos are fetched from their release repositories. Before compiling them their dependencies are being installed from Debian packages and this is when one of them fails to install. And it fails since that package never built before and does not exist in the Debian repository: http://www.ros.org/debbuild/hydro.html?q=gazebo_taskboard