ora-28 session killed message in alert log

issue:
alert log displays error message:
opiodr aborting process unknown ospid (24094) as a result of ORA-28
Mon Jul 16 14:23:59 2012
opiodr aborting process unknown ospid (16788) as a result of ORA-28

explanation:
the ora-28 error is just message notifucation error. it is generated when privilege user kills session.

reference:
http://jzhil2004.blog.163.com/blog/static/2755850420111115293348/