xfce4-session - Known Issues
Shutting down your computer using the session manager
Since 4.1.5, the session manager supports only sudo(8)-based shutdown, other method can be added by packagers if desired, just replace XfsmShutdownHelper with your code. To be able to shutdown the computer, you must be listed in the systems sudoers file, in particular, you must be allowed to execute $HELPER_PATH_PREFIX/xfce4/session/xfsm-shutdown-helper as user root (where $HELPER_PATH_PREFIX is the directory passed to configure with the –with-helper-path-prefix option or /usr/local/lib by default).
For example, you built xfce4-session with the default options, your hostname is myhost and your user account is named myuser, then you'd have to add the following line to your sudoers file (remember to use visudo to edit that file):
myuser myhost=/usr/local/lib/xfce4/session/xfsm-shutdown-helper
Starting with xfce4-session 4.3.99.2 xfce4-session will first try to use HAL (if built with D-Bus support) and fallback to the sudo method described above. So if you have HAL installed on your system, and your account is allowed to shutdown/reboot the computer using HAL, you don't need to setup sudo at all.
Legacy session management
As of version 4.1.7, the session manager includes native support for applications that don't support the X11R6 session management protocol, but support legacy (X11R5) session management. If you don't need legacy session management support, you can disable it at compile time, giving –disable-legacy-sm to ./configure.
The legacy sm code is ment as a replacement for smproxy (the X11 session
management proxy), which caused all kinds of trouble. That says, do NOT
EVER run smproxy in session that is managed by xfce4-session, or weird
things will happen. The included legacy session management does everything
that smproxy would do, and besides that, it also support multiscreen
display and it is less error prone compared to smproxy (atleast once
the various bugs I introduced are fixed 
.
Known issues when installing xfce4-session 4.13 from source:
Installing xfce4-session from source possibly may conflict with the distribution package installation of xfce4-session 4.12, leading to a crash during startup. When starting the self build xfce4-session from console, the following error is given: Gtk-Error **: GTK+ 2.x symbols detected. Using GTK+ 2.x and GTK+ 3 in the same process is not supported
It looks like the cause of the problem is, that libgtk-x11-2.0 and libgdk-x11-2.0 were linked.
However it is not fully clear why, and how it can be prevented (uninstall gtk2-dev ? )
it looks like the problem can be solved by removing xfce4-session which was installed by the distribution, re build from source and re-installing the self build xfce4-session. Note that on debian based systems it is required to use “apt-get –purge xfce4-session”. –remove is not sufficient.
Here some more information on it: https://bugzilla.xfce.org/show_bug.cgi?id=15104