This has been reported on Ubuntu Launchpad as: 
https://bugs.launchpad.net/bugs/466297

From a duplicate - https://bugs.launchpad.net/bugs/526833

 (Steps to reproduce:
1. start vpnc connection to remote network

2. Run remote desktop viewer to a RHEL Gnome desktop

3. Xfce4 crashes

From the original launchpad reporter:

ProblemType: Crash
Architecture: i386
AssertionMessage: xfce4-xfapplet-plugin: ../../src/xcb_io.c:378: _XAllocID: Assertion `ret != inval_id' failed.
CrashCounter: 1
Date: Sat Oct 31 01:35:06 2009
DistroRelease: Ubuntu 9.10
ExecutablePath: /usr/lib/xfce4-xfapplet-plugin/xfce4/panel-plugins/xfce4-xfapplet-plugin
Package: xfce4-xfapplet-plugin 0.1.0-3ubuntu4
ProcCmdline: /usr/lib/xfce4-xfapplet-plugin/xfce4/panel-plugins/xfce4-xfapplet-plugin socket_id 31457321 name xfapplet id 12519606270 display_name XfApplet size 50 screen_position 11
ProcEnviron:
 LANGUAGE=en_US.UTF-8
 PATH=(custom, no user)
 LANG=en_US.UTF-8
 SHELL=/bin/bash
ProcVersionSignature: Ubuntu 2.6.31-14.48-generic
Signal: 6
SourcePackage: xfce4-xfapplet-plugin
StacktraceTop:
 __kernel_vsyscall ()
 raise () from /lib/tls/i686/cmov/libc.so.6
 abort () from /lib/tls/i686/cmov/libc.so.6
 __assert_fail () from /lib/tls/i686/cmov/libc.so.6
 _XAllocID () from /usr/lib/libX11.so.6
Title: xfce4-xfapplet-plugin assert failure: xfce4-xfapplet-plugin: ../../src/xcb_io.c:378: _XAllocID: Assertion `ret != inval_id' failed.
Uname: Linux 2.6.31-14-generic i686
UserGroups:

XsessionErrors:
 (xfce4-mixer-plugin:867): libxfce4mixer-CRITICAL **: xfce_mixer_get_track: assertion `GST_IS_MIXER (card)' failed
 (polkit-gnome-authentication-agent-1:917): GLib-CRITICAL **: g_once_init_leave: assertion `initialization_value != 0' failed

Stacktrace: 
http://launchpadlibrarian.net/35243120/Stacktrace.txt

ThreadStacktrace: 
http://launchpadlibrarian.net/35243121/ThreadStacktrace.txt