We recently discovered that the Jolokia agent is incompatible with the -XX:+PerfDisableSharedMem JVM option. I assume that this may affect other monitoring tools as well.
If we are going to leave this enabled by default, we should document the potential problems with it. A combination of a comment in cassandra-env.sh (and the Windows equivalent) and a comment in NEWS.txt should suffice, I think.
If possible, it would be good to figure out what other tools are affected and also mention them.