Version:            (using KDE 4.2.2)
OS:                Linux
Installed from:    SuSE RPMs

Using a multi desktops configuration.
If you open the Application Launcher menu it only displays in one desktop. If you switch desktop while it is open, then you  have to click 2 times to make it appears on this other desktop.

If you add the system monitor widget to the Panel you will not be able to open it unless, 
- the application launcher is closed (in any desktop) or
- the application launcher is open on the same desktop.

The reverse is true if the system monitor is open on one desktop then you switch desktop, you won't be able to open the Application Launcher menu on this other desktop.
This is especially annoying since you have to hunt done the other desktop to found the one on which the System Monitor is opened.

Playing with desktop switching, you can reach a situation were you cannot anymore open the Application Launcher and/or the System Monitor. The only solution is to switch to classical menu style for the Application Launcher and to  remove then re-add the System Monitor Widget.