The menus such as Applications, Places, and System, as well as tooltips, are shared between all virtual desktops instead of confined to the current one.
Steps to reproduce the problem:
> Enable Compiz on a stock Ubuntu installation
> Use CCSM or configuraiton tool of choice to turn the desktop into a 5-desktop "cube" that is transparent even when not rotating.
> Activate a menu on the active desktop, or scroll the mouse over a widget that has a tooltip in an open program.
Expected behaviour:
> The tooltip or menu shows up only on the current desktop.
Actual behaviour:
> The menu shows up many times, giving the illusion that activity is happening on the other desktops.
> Tooltips show up on other desktops, even when the application that those tooltips serve is not shown on those desktops. Thus, the tooltip appears to be floating out of context.
Thanks for taking the time to help Ubuntu getting better. I think what you reported is not a bug because the same behavior can be observed for any sticky (i.e. shown on all desktops) window. I'm marking this bug as invalid for now, but feel free to discuss it in the Answers question if you don't feel OK with this feature.