Comment 1 for bug 876290

This may not be so much a shortcut bug, as a bug between the development of the log out/shutdown script.
If you go to your menu click more apps, filter to system apps, and look through your list of installed apps you will find that log out and shut down are two different applications(well the way they treat them at least). They can not be one dialog. But now that you do mention it, it is weird that they would create this application in this way.

A quick fix for you, would be to go to System Settings -> Keyboard -> Shortcuts -> Custom Shortcuts
then make a new shortcut called shutdown
and in the line asking for the command enter "/usr/lib/indicator-session/gtk-logout-helper -s"
without the quotes, then set your shortcut to what ever you desire
hope this helps for now