File this one under the misleading-errors department. One of my developers was working with a new queue. He pinged me when he got this error trying to create a job that used the queue:

ERROR at line 1:
ORA-27373: unknown or illegal event source queue
ORA-06512: at "SYS.DBMS_ISCHED", line 124
ORA-06512: at "SYS.DBMS_SCHEDULER", line 314
ORA-06512: at line 2

The CREATE_JOB statement was:

BEGIN
DBMS_SCHEDULER.CREATE_JOB(
job_name => 'foo.bar_q_job',
job_type => 'PLSQL_BLOCK',
job_action => 'begin foo.bar_pkg.consume_bar_queue(); end;',
queue_spec => 'BAR.BAR_Q, FOO_BAR_AGENT',
enabled => true,
comments => 'This is a job to consume the bar.bar_q entries that affect foo.');
END;
/

After a few minutes of banging our heads, it became obvious that this was a permissions problem. The queue was owned by BAR, the job was being created as FOO. The ORA error message could/should have made this more obvious, in my opinion.

Anyway, the fix was simply to grant access to FOO:

DBMS_AQADM.GRANT_QUEUE_PRIVILEGE(
        privilege  => 'ALL',
        queue_name => 'bar.bar_q',
        grantee    => 'foo' );

Hope this saves some banged heads for others.