In my Keyboard > Application Shortcuts control panel, I normally set the Control+Print or Shift+Print key combination to launch xfce4-screenshooter -w. This works at first, but after rebooting my system, they key combination no longer functions (though it still appears in the Application Shortcuts) list. I'm using Xubuntu 12.04 (precise) xfce4-settings package version 4.8.3-1ubuntu3
It seems that my Control+Alt+Delete shortcut is being ignored as well, despite the fact I have mapped it to launch xflock4. Removing and re-creating the shortcut doesn't fix it. Reset To Defaults and then re-creating the shortcut doesn't fix it either.
My Alt+F2 shortcut always works, though. It looks like the bug might have something to do with the Control key.
Should be fixed in the current master branches of libxfce4ui, xfce4-settings and xfwm4. Feel free to reopen if you can still reproduce.