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:

When you start with Oracle Data Guard you have three sources of useful information: The Data Guard Concepts and Administration manual, which is quite good, other peoples experience, and after some time your own. The manual does not include all the mistakes one can do, nor what many really don’t use (like the data guard broker; not everybody would leave switchover, or worse failover to a robot).