This bug has been reported on Ubuntu Launchpad as:
https://launchpad.net/bugs/554661

Binary package hint: xfce4-panel

i've tried to shutdown my system and the panel crashed

DistroRelease: Ubuntu 10.04

Package: xfce4-panel 4.6.3-1ubuntu2

Uname: Linux 2.6.32-19-generic i686
Architecture: i386
ExecutablePath: /usr/bin/xfce4-panel
ProcCmdline: xfce4-panel --sm-client-id 247fd80f9-ae57-4140-8498-6c9f5654daa3
SegvAnalysis:
 Segfault happened at: 0x46219b: cmp %cl,(%edx)
 PC (0x0046219b) ok
 source "%cl" ok
 destination "(%edx)" (0x00000001) not located in a known VMA region (needed writable region)!
SegvReason: writing NULL VMA
Signal: 11

StacktraceTop:
__strcmp_ssse3 () at ../sysdeps/i386/i686/multiarch/strcmp-ssse3.S:77
 ?? () from /usr/lib/libgobject-2.0.so.0
 g_hash_table_lookup () from /lib/libglib-2.0.so.0
 g_param_spec_pool_lookup ()
 g_object_new_valist () from /usr/lib/libgobject-2.0.so.0

XsessionErrors:
 (polkit-gnome-authentication-agent-1:1763): GLib-CRITICAL **: g_once_init_leave: assertion `initialization_value != 0' failed
 (Do:1735): Wnck-CRITICAL **: wnck_set_client_type got called multiple times.
 (gnome-terminal:2043): Gtk-CRITICAL **: gtk_accel_map_unlock_path: assertion `entry != NULL && entry->lock_count > 0' failed
 (xfce4-panel:1725): libxfcegui4-WARNING **: ICE I/O Error
 (xfce4-panel:1725): libxfcegui4-WARNING **: Disconnected from session manager.

Stacktrace attachment: http://launchpadlibrarian.net/43457895/Stacktrace.txt