Oracle Enterprise Manager (EM) reports on many security policies when they are violated. One of them are about execute privileges on standard PL/SQL packages granted to PUBLIC. One a day this summer with nothing better to do I went on to revoke these privileges. The wages for my efforts was several  ORA-600 errors:

Case: I wanted to reinstall the Enterprise Manager agent on a host. I went on to delete the host in the list of targets in EM. Then after a new installation and synchronization I somehow managed to push the list of targets from GC to the agent and thereby removing the host as a target in the local targets.xml file. Result: The monitored host did never show up in the target list of hosts even after a successful installation of the agent.