In testing, very rarely we see Riak fail to shutdown. Nothing is still running, but the process is still hanging around.
This problem was also seen in the wild and reported on the riak-users list: http://lists.basho.com/pipermail/riak-users_lists.basho.com/2013-May/012019.html
One possible hint at a fix was mentioned on the erlang-bugs list: http://erlang.org/pipermail/erlang-bugs/2013-May/003587.html