I use xfce4-panel within a gnome session (it handles xrandr changes better than the native gnome-panel).
Whenever I exit the X session with Ctrl+Alt+Backspace or through tty (with "gdm stop"), the applets loaded through xfapplet keep running (I can confirm it through a simple "ps x" inspection), and some can't restart when the X session restarts (usually volume control, but happened already with deskbar applet and intlclock).
I can avoid this by manually killing each xfapplet instance and restarting X session. I find no error message upon inspecting many log files.
I am running Ubuntu 8.04 (Hardy) in x86, xfce4-panel version is 4.4.2-3ubuntu3 and xfce4-xfapplet-plugin version is 0.1.0-3ubuntu2.
Thank you