Comment 27 for bug 740767

With Natty, alt-space worked for several weeks after installation then stopped, showing the symptoms as described above. Just clicking on the "use default" button on the right of the corresponding field in the compiz configuration manager restored the correct value into the box (/usr/bin/compiz-decorator). The change took effect after a X-windows restart with ctrl-alt-backspace. The modification persisted throughout x-restarts and logout/logins.

Also, here is the way I was able to reproduce the unintended modification of the Command value to its buggy state:

1. From Ubuntu Software Center install Compiz Fusion Icon.
2. Start the applet from Applications->System Tools->Compiz Fusion Icon.
3. Right-click on the blue arrowed box icon that has appeared in the notification area.
4. From Select window manager menu item select Metacity (I also have Openbox listed).
5. Check the Compiz Configuration Manager. The value of the Command field is now "gtk-windows-decorator --replace" .
6. From the Compiz Fusion Icon select back Compiz.
7. The value of the Command field does not return back to "/usr/bin/compiz-decorator" and alt-space keep crashing.

After several tries changing the windows manager stopped changing the Command value, until I restarted the Compiz Fusion Icon applet. After that I was able to reproduce it again.