keyboard shortcuts doesn't always do what it promises

In System Settings -> Keyboard -> Shortcuts, sometimes shortcuts don't take effect. (Perhaps something else is seeing them first?) For example, I set "Show the run command prompt" to Ctrl+Alt+R, but pressing C-A-R doesn't have any noticeable effect. Moreover, there's no obvious way to find out what command is being run in response to the key press, so I can't debug at the level of "does the command run on my system when invoked form a terminal".

The keyboard shortcuts app does remember that this is the keyboard shortcut I've requested, it just doesn't result in a keyboard shortcut.

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: unity 5.10.0-0ubuntu6
ProcVersionSignature: Ubuntu 3.2.0-24.37-generic 3.2.14
Uname: Linux 3.2.0-24-generic x86_64
ApportVersion: 2.0.1-0ubuntu7
Architecture: amd64
CompizPlugins: [core,bailer,detection,composite,opengl,compiztoolbox,decor,gnomecompat,grid,move,mousepoll,resize,vpswitch,imgpng,wall,place,unitymtgrabhandles,snap,regex,animation,session,workarounds,fade,scale,expo,ezoom,unityshell]
Date: Thu May 3 09:07:23 2012
EcryptfsInUse: Yes
InstallationMedia: Ubuntu 10.04 LTS "Lucid Lynx" - Release amd64 (20100429)
ProcEnviron:
 TERM=xterm
 PATH=(custom, user)
 LANG=en_GB.UTF-8
 SHELL=/bin/bash
SourcePackage: unity
UpgradeStatus: Upgraded to precise on 2012-05-03 (0 days ago)