Revoke privileges from PUBLIC may cause ORA-600

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:

ORA-00600: internal error code, arguments: [qmxqtmChkXQAtomMapSQL:2], [], [], [], [], [], [], [], [], [], [], []
00600. 00000 -  "internal error code, arguments: [%s], [%s], [%s], [%s], [%s], [%s], [%s], [%s]"
*Cause:    This is the generic internal error number for Oracle program
           exceptions.     This indicates that a process has encountered an
           exceptional condition.
*Action:   Report as a bug - the first argument is the internal error number

A quick research on My Oracle Support indicated that some standard packages depends on these privileges granted to PUBLIC. The solution is to grant back to PUBLIC if you are in a hurry, or grant the privilege to the owners of the individual packages (like XDB). The errors continued after the granting was done, but stopped after I bounced the database instance.

Just because EM reports something and the solution seems obvious, it is not alway wise to apply remedy immediately, especially in production. It surprised me a little that EM suggests a security policy that Oracle's own packages do not adhere to.