user@host:~$ gdb --args xfce4-panel --sm-client-id=5071 --sync
GNU gdb (GDB) 7.2
Copyright (C) 2010 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-unknown-linux-gnu".
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>...
Reading symbols from /usr/bin/xfce4-panel...done.
(gdb) run
Starting program: /usr/bin/xfce4-panel --sm-client-id=5071 --sync
[Thread debugging using libthread_db enabled]
[New Thread 0x7fffeece3700 (LWP 12872)]
(xfce4-mixer-plugin:12871): xfce4-mixer-plugin-DEBUG: mixer_plugin->track_label = 'Headphone'
(xfce4-mixer-plugin:12873): xfce4-mixer-plugin-DEBUG: mixer_plugin->track_label = 'Speaker'
(xfce4-mixer-plugin:12874): xfce4-mixer-plugin-DEBUG: mixer_plugin->track_label = 'Hauptregler'
[New Thread 0x7fffe7358700 (LWP 12894)]
[New Thread 0x7fffe6b57700 (LWP 12895)]
[New Thread 0x7fffe6356700 (LWP 12896)]
[New Thread 0x7fffe5b55700 (LWP 12897)]
[Thread 0x7fffe5b55700 (LWP 12897) exited]
[Thread 0x7fffe7358700 (LWP 12894) exited]
[Thread 0x7fffe6b57700 (LWP 12895) exited]
[Thread 0x7fffe6356700 (LWP 12896) exited]
[New Thread 0x7fffe6356700 (LWP 12903)]
[New Thread 0x7fffe6b57700 (LWP 12904)]
[New Thread 0x7fffe7358700 (LWP 12905)]
[Thread 0x7fffe7358700 (LWP 12905) exited]
[Thread 0x7fffe6356700 (LWP 12903) exited]
[Thread 0x7fffe6b57700 (LWP 12904) exited]

libtasklist-CRITICAL **: tasklist-widget.c:879 (xfce_tasklist_size_layout): expression 'g_slist_length (windows_scored) == (guint) tasklist->n_windows' failed.
aborting...

Program received signal SIGABRT, Aborted.
0x00007ffff4cb6675 in raise () from /lib/libc.so.6
(gdb) continue
Continuing.

Program received signal SIGABRT, Aborted.
0x00007ffff4cb6675 in raise () from /lib/libc.so.6
(gdb) continue
Continuing.
[Thread 0x7fffeece3700 (LWP 12872) exited]

Program terminated with signal SIGABRT, Aborted.
The program no longer exists.
(gdb) quit