This may be related to the bug logged in: http://sourceware.org/bugzilla/show_bug.cgi?id=2921 I'm seeing repeatable situations where just opening a new session / or accessing an old session causes the exception in http://sourceware.org/bugzilla/show_bug.cgi?id=2921, after which the Gnome panel is hung.
This ia duplicate of 2921. The syscall observers is faulty and because of that the gnome-panel tasks gets left in a blocked state. *** This bug has been marked as a duplicate of 2921 ***